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{{Hatnote|"WP:CATEGORY" and "WP:CAT" redirect here. You may be looking for [[Wikipedia:WikiProject Cats|WikiProject Cats]], [[Wikipedia:HotCat]], or [[Wikipedia:WikiProject Categories|WikiProject Categories]]. For the category system itself, see [[:Category:Contents]].}}
{{Subcat guideline|editing guideline|Categorization|WP:CAT|WP:CATEGORY|WP:CG|WP:MOSCAT}}
{{Guideline list}}
This page contains [[Wikipedia:Policies and guidelines |guidance]] on the proper use of the '''categorization''' function in Wikipedia. For information on the mechanics of the function, category syntax, etc., see '''[[Help:Category]]'''. For quick answers, see the '''[[Wikipedia:FAQ/Categorization|Categorization FAQ]]'''. For proposals to delete, merge, or rename categories, follow the instructions at [[Wikipedia:Categories for discussion|Categories for discussion]]. Please use it before undertaking any complicated re-categorization of existing categories or mass creation of new categories.
The central goal of the category system is to provide navigational links to Wikipedia pages in a hierarchy of ''categories'' which readers, knowing essential—''defining''—characteristics of a topic, can browse and quickly find sets of pages on topics that are defined by those characteristics.
Categories are not the only means of enabling users to browse sets of related pages. Other tools which may be used instead of or alongside categories in particular instances include lists and navigation boxes. For a comparison of these techniques, see [[Wikipedia:Categories, lists, and navigation templates|Categories, lists and navigation templates]].
== Naming conventions ==
{{Further|Wikipedia:Category names}}
{{Wikipedia:Categorization/Naming}}
==Creating category pages==
{{Also|Wikipedia:HotCat}}
After you have determined an appropriate category name ''and'' know its parent category, you are ready to create the new category. To create a category, first add an article to that category. Do this by editing the article page. At the bottom, but before the [[H:ILL|interwiki links]] (if any are present), add the name of the new category, (e.g.: {{code|[[Category:New category name]]}} ), and save your edit. The as-yet-undefined category name will now appear as a red link in the article's category list at the bottom of the page.
Next, to create the category, click on that red link, which brings you back into the editor. Adding this new category into the appropriate parent category is much the same as with an article: at the bottom, simply add the parent category (e.g.: {{code|[[Category:Parent category name]]}} ), which should usually be a [[hypernym]] of the sub-category.
Sometimes, a common-sense guess based on the title of the category isn't enough to figure out whether a page should be listed in the category. So, rather than leave the text of a category page empty (containing only parent category declarations), it is helpful – to both readers and editors – to include a description of the category, indicating what pages it should contain, how they should be subcategorized, and so on.
In such cases, the desired contents of the category should be described on the category page, similar to how the [[WP:LSC|list selection criteria]] are described in a stand-alone list. The category description should make direct statements about the criteria by which pages should be selected for inclusion in (or exclusion from) the category. This description, not the category's name, defines the proper content of the category. Do not leave future editors to guess about what or who should be included from the title of the category. Even if the selection criteria might seem obvious to you, an explicit standard is helpful to others, especially if they are less familiar with the subject.
The description can also contain links to other Wikipedia pages, in particular to other related categories which do not appear directly as subcategories or parent categories, and to relevant categories at [[Wikipedia:Wikimedia sister projects|sister projects]], such as [[Wikipedia:Commons|Commons]]. Another technique that can be used is described at [[Wikipedia:Classification]]. Like [[Wikipedia:Disambiguation|disambiguation]] pages, category pages should not contain either citations to [[Wikipedia:Identifying reliable sources|reliable sources]] or [[Wikipedia:External links|external links]].
Various templates have been developed to make it easier to produce category descriptions; see [[:Category:Category namespace templates|Category namespace templates]]. There are hatnote templates including {{tl|Cat main}} and {{tl|Category see also}}; others are listed at [[Wikipedia:Hatnote#Categories]].
A maximum of 200 category entries are displayed per screen. To make navigating large categories easier, a table of contents can be used on the category page. The following templates are some of the ways of doing this:
* {{Tl|Category TOC}} – adds a complete table of contents (Top, 0–9, A–Z)
* {{Tl|Large category TOC 2}} – adds a complete table of contents with five subdivisions for each letter (Aa Ae Aj Ao At)
* {{Tl|Large category TOC}} – adds a complete table of contents with twenty-six subdivisions for each letter (Aa ... Az)
Subcategories are split alphabetically along with the articles, which means that the initial screen of a split category may not include all its subcategories. To make all subcategories display on each screen, add a category tree to the text of the category page, as described at the help page under [[Help:Category#Displaying category trees and page counts|Displaying category trees and page counts]].
Category pages can have [[Help:Interlanguage links|interlanguage links]] in the "Languages" list in the left sidebar (in the default skin), linking to corresponding categories in other language Wikipedias. To edit these on [[Wikidata]], click on the "Edit links" link at the end of the languages list.
==Categorizing pages==
[[File:Category explanation.ogv|thumb|A screencast showing how to categorise pages and explaining the usage of [[WP:HOTCAT|HotCat]]]]
{{see also|Wikipedia:Categorization dos and don'ts|Wikipedia:Categorizing redirects}}
Every Wikipedia page should belong to at least one category. (However, there is no need to categorize [[Wikipedia:Talk page|talk pages]], [[Wikipedia:Redirect|redirects]], or [[Wikipedia:User page|user pages]], though these may be placed in categories where appropriate.) In addition, each categorized page should be placed in all of the ''most specific'' categories to which it logically belongs. This means that if a page belongs to a [[#Subcategorization|subcategory]] of C (or a subcategory of a subcategory of C, and so on) then it is ''not'' normally placed directly into C. For exceptions to this rule, see [[#Eponymous categories|Eponymous categories]] and [[#Non-diffusing subcategories|Non-diffusing subcategories]] below.
While it should typically be clear from the name of an existing category which pages it should contain, the text of the category page may sometimes provide additional information on potential category contents. One way to determine if suitable categories already exist for a particular page is to check the categories of pages concerning similar or related topics. Another way is to search existing category names as described [[Help:Searching|here]] (top of page). Since all categories form part of a hierarchy, do not add categories to pages as if they are [[Tag (metadata)|tags]].
===Articles===
{{see also|Wikipedia:Categorization of people}}
{{For|category types to avoid|Wikipedia:Overcategorization}}
{{anchor|Verifiable|Verifiability}}{{Shortcut|WP:CATV|WP:CATVER}}
Categorization of articles must be '''[[Wikipedia:Verifiability|verifiable]]'''. It should be clear from verifiable information in the article why it was placed in each of its categories. Use the {{tl|Category unsourced}} template if you find an article in a category that is not shown by sources to be appropriate or if the article gives no clear indication for inclusion in a category.
Categorization must also maintain a '''[[Wikipedia:NPOV|neutral point of view]]'''. Categorizations appear on article pages without annotations or referencing to justify or explain their addition; editors should be conscious of the need to maintain a neutral point of view when creating categories or adding them to articles. Categorizations should generally be uncontroversial; if the category's topic is likely to spark controversy, then a list article (which can be annotated and referenced) is probably more appropriate. For example, a politician (not convicted of any crime) should not be added to a category of notable criminals.
{{Anchor|Defining characteristics|Defining}}{{Shortcut|WP:CATDEF|WP:CATDEFINING}}<section begin=DefiningDef /><section begin=DefiningFull />A central concept used in categorising articles is that of the '''[[WP:DEFINING|defining]]''' characteristics of a subject of the article. A defining characteristic is one that [[WP:RS|reliable sources]] ''commonly'' and ''consistently'' define<ref>in prose, as opposed to a tabular or list form</ref> the subject as having—such as nationality or notable profession (in the case of people), type of location or region (in the case of places), etc.<section end=DefiningDef /> For example, here: "Caravaggio, an Italian artist of the Baroque movement ...", ''Italian'', ''artist'', and ''Baroque'' may all be considered to be defining characteristics of the subject ''Caravaggio''.<section end=DefiningFull />
Particular considerations for categorizing articles:
{{Shortcut|MOS:CATORDER}}
* By convention, category declarations are placed at the end of the wikitext, but before any [[Wikipedia:Stub|stub]] templates (which themselves transclude categories).
*{{Anchor|CATORDER}} [[WP:EPONYMOUS|Eponymous categories]] should appear first. Beyond that, the order in which categories are placed on a page is not governed by any single rule (for example, it does not need to be alphabetical, although partially alphabetical ordering can sometimes be helpful). Normally the most essential, significant categories appear first.
* An article should never be left with a non-existent ([[Wikipedia:REDLINK|redlinked]]) category on it. Either the category should be created, or else the link should be removed or changed to a category that does exist.
* Categorization should not be made by the ''type'' of an article. A biographical article about a specific person, for example, does not belong in [[:Category:Biography (genre)]].
* Articles on fictional subjects should not be categorized in a manner that confuses them with real subjects. {{tl|Category see also}} is useful for interlinking examples of real-world and fictional phenomena
==== Eponymous categories ====
{{Shortcut|WP:EPON|WP:EPONYMOUS}}
{{see also|Category:Eponymous categories|Wikipedia:Overcategorization#Eponymous categories for people}}
A category that covers exactly the same topic as an article is known as the '''eponymous category''' for that article (e.g. [[New York City]] and [[:Category:New York City]]; [[Mekong]] and [[:Category:Mekong River]]).
An eponymous category should have only the categories of its article that are relevant to the category's content. For example:
*Both [[New York City]] and [[:Category:New York City]] fit well in [[:Category:Cities in New York (state)]].
*The article [[New York City]] is in [[:Category:Populated places established in 1624]], but this category is not necessarily relevant to the content of [[:Category:New York City]], so it should not be used on the eponymous category.
'''Guidelines for articles with eponymous categories'''
*The article itself should be a member of the eponymous category and should be sorted with a space to appear at the start of the listing (see [[#Sort keys]]).
*The article should be listed as the main article of the category using the {{tl|cat main}} template.
*Articles with an eponymous category may be categorized in the broader categories that would be present if there were no eponymous category (e.g. the article [[France]] appears in both [[:Category:France]] and [[:Category:Western Europe]], even though the latter category is the parent of the former category). Editors should decide by consensus which solution makes most sense for a category tree. There are three options:
# Keep both the eponymous category and the main article in the parent category. This is used in [[:Category:Western Europe]] to allow that region's country articles to be navigated together.
# Keep just the child article. This is used in [[:Category:British Islands]], to prevent a loop.
# Keep just the eponymous category. This is used for [[:Category:Farmworkers]] in [[:Category:People by occupation]]. Such "''X'' by ''Y''" categories sometimes cover a limited navigational set, not a topic (see [[#Category tree organization]]), thus there is no logical article content.
If eponymous categories are categorized separately from their articles, it will be helpful to make links between the category page containing the articles and the category page containing the eponymous categories. The template {{Tl|Related category}} can be used for this. An example of this set-up is the linked categories [[:Category:American politicians]] and [[:Category:Wikipedia categories named after American politicians]].
==={{anchor|Images}}Files/images===
{{shortcut|WP:FILECAT}}
{{see|Wikipedia:Manual of Style/Images#Image description pages}}
Category tags can be added to [[Help:Files|file]]/image pages of files that have been uploaded to Wikipedia. When categorized, files are not included in the count of articles in the category, but are displayed in a separate section with a thumbnail and the name for each. A category can mix articles and images, or a separate file/image category can be created. A file category is typically a subcategory of the general category about the same subject, and a subcategory of the wider category for files, [[:Category:Wikipedia files]]. To categorize a new file when uploading, simply add the category tag to the [[Help:Edit summary#File upload summary|upload summary]].
Freely licensed files may also be uploaded to, and categorized on, [[WP:COMMONS|Wikimedia Commons]]. This can be done instead of, or in addition to, uploading and categorizing on Wikipedia. Most freely licensed files will eventually be copied or moved from Wikipedia to Commons, with a mirror page remaining on Wikipedia. (For an example of one such mirror page, see [[:File:Niederwald memorial 2.JPG|here]].) Categories should not be added to these Wikipedia mirror pages, because doing so creates a new Wikipedia page that is subject to [[WP:SD#F2|speedy deletion]]. Exceptions to this principle are made for mirror pages of images that are nominated as [[Wikipedia:Featured pictures|featured pictures]] and for those that appear on the Wikipedia [[Main Page]] in the [[:Template:Did you know|Did You Know?]] column.
Images that are used in Wikipedia that are [[Wikipedia:Non-free content|non-free or fair use]] should not appear as thumbnail images in categories. To prevent the thumbnail preview of images from appearing in a category, <nowiki>__NOGALLERY__</nowiki> should be added to the text of the category. In such cases, the file will still appear in the category, but the actual image preview will not.
==={{anchor|Non-article and maintenance categories|Maintenance categories}} Wikipedia administrative categories ===
{{Shortcut|WP:PROJCATS}}
{{see|Wikipedia:Administration#Data structure and development}}
A distinction is made between two types of categories:
* '''Administrative categories''', intended for use by editors or by automated tools, based on features of the current state of articles, or used to categorize non-article pages.
* '''Content categories''', intended as part of the encyclopedia, to help readers find articles, based on features of the subjects of those articles;
Administrative categories include '''stub categories''' (generally produced by [[Wikipedia:Stub|stub]] templates), '''maintenance categories''' (often produced by tag templates such as {{tl|cleanup}} and {{tl|fact}}, and used for [[Wikipedia:Maintenance|maintenance]] projects), '''[[Wikipedia:WikiProject|WikiProject]] and [[Wikipedia:Assessment|assessment categories]]''', and '''categories of pages in non-article [[Wikipedia:Namespace|namespaces]]'''.
Article pages should be kept out of administrative categories if possible. For example, the templates that generate WikiProject and assessment categories should be placed on talk pages, not on the articles themselves. If it is unavoidable that an administration category appears on article pages (usually because it is generated by a maintenance tag that is placed on articles), then in most cases it should be made a '''hidden category''', as described in [[#Hiding categories|§ Hiding categories]] below.
There are separate administrative categories for different kinds of non-article pages, such as [[Wikipedia:Template|template]] categories, [[Wikipedia:Disambiguation|disambiguation page]] categories, [[Wikipedia:Wikipedia namespace|project page]] categories etc.
==={{anchor|User namespace|Categorizing user pages}} User pages ===
{{shortcut|WP:USERNOCAT}}
{{Further|Wikipedia:User categories|Wikipedia:User pages#Categories, templates that add categories, and redirects}}
[[Wikipedia:User pages|User pages]] are not articles, and thus do not belong in ''content categories'' such as [[:Category:Living people|Living people]] or [[:Category:Biologists|Biologists]]. They can however be placed in '''user categories''' – subcategories of [[:Category:Wikipedians]], such as [[:Category:Wikipedian biologists]] – which assist collaboration between users.
Similarly, [[WP:SP|user subpages]] that are draft versions of articles should be kept out of content categories, but are permitted in non-content or project categories, like [[:Category:User essays]]. If you copy an article from mainspace to userspace and it already contains categories, remove them or [[Help:Hidden text|comment them out]]. Restore the categories when you move the draft back into article space.
At [[WP:Database reports/Polluted categories|Database reports/Polluted categories]], a list of affected categories is maintained.
=== Draft pages ===
{{shortcut|WP:DRAFTNOCAT}}
{{Further|Wikipedia:Drafts#Preparing drafts}}
Pages in the [[Wikipedia:Drafts|draft namespace]] are not articles, and thus do not belong in content categories such as [[:Category:Living people|Living people]] or [[:Category:Biologists|Biologists]]. <!-- They may be placed in ???? --> If you copy an article from mainspace to draftspace and it already contains categories, then disable them: edit each by inserting a colon character to [[Wikipedia:How to link to a category|link to its category]] instead (e.g. change <code><nowiki>[[Category:Biologists]]</nowiki></code> to <code><nowiki>[[:Category:Biologists]]</nowiki></code>), or [[Help:Hidden text|comment them out]] (e.g., <code><nowiki><!-- [[Category:Biologists]] --></nowiki></code>), or insert "nowiki" tags (e.g., {{tag|nowiki|content=<nowiki>[[Category:Biologists]]</nowiki>}}). Re-enable the categories when you move the draft back into article space.
==={{Anchor|T}}Template categorization===
{{shortcut|WP:CAT#T}}
[[Wikipedia:Template namespace|Templates]] should be categorized according to kind of template, but not by template content. For example, [[:Template:Schubert string quartets]] is categorized under [[:Category:String quartets by composer templates]], which should be a subcategory of [[:Category:Music navigational boxes]] (kind) but [[:Template:Schubert string quartets]] should ''not'' be categorized under [[:Category:Franz Schubert]] or [[:Category:String quartets]] (content).
It is usually desirable that pages using a template are not placed in the same categories as the template itself. To avoid this, the category for the template should be placed on the template's documentation page, normally within a {{tag|includeonly}} block; if there is no documentation page, the category for the template may be placed on the template itself, within a {{tag|noinclude}} block. Where a {{tag|noinclude}} block is the last item in the template code, there should be no spaces or newlines between the last part of the template proper and the opening {{tag|noinclude|o}} tag.
===Categorization using templates===
{{shortcut|WP:TEMPLATECAT|WP:TCAT}}
Many [[Wikipedia:Template messages|templates]] include category declarations in their transcludable text, for the purpose of [[:Category:Tracking categories|placing the pages containing those templates into specific categories]]. This technique is very commonly used for populating certain kinds of administration categories, including stub categories and maintenance categories.
However, it is recommended that articles not be placed in ordinary ''content'' categories using templates in this way. There are many reasons for this: editors cannot see the category in the wikitext; removing or restructuring the category is made more difficult (partly because automated processes will not work); inappropriate articles and non-article pages may get added to the category; sort keys may be unavailable to be customised per category; and ordering of categories on the page is less controllable.
When templates are used to populate administration categories, ensure that the code cannot generate nonsensical or non-existent categories, particularly when the category name depends on a parameter. Also, see [[Wikipedia:Category suppression|Category suppression]] for ways of keeping inappropriate pages out of template-generated categories.
Category declarations in templates often use <code><nowiki>{{PAGENAME}}</nowiki></code> as the sort key, because this overrides any DEFAULTSORT defined on the page.
====Hiding categories====
{{Shortcut|WP:HIDDENCAT}}
In cases where, for technical reasons, administration categories appear directly on articles rather than talk pages, they should be made into '''hidden categories''', so that they are not displayed to readers. This rule does not apply to [[Wikipedia:Stub|stub]] categories or "uncategorized article" categories – these types are ''not'' hidden.
To hide a category, add the template {{tlx|Wikipedia category|<nowiki>hidden=yes</nowiki>}} to the category page (the template uses the [[Wikipedia:Magic word|magic word]] <code><nowiki>__HIDDENCAT__</nowiki></code>). This also places the page in [[:Category:Hidden categories]].
A logged-in user may elect to view all hidden categories, by checking "Show hidden categories" on the "Appearance" tab of [[Special:Preferences|Preferences]]. Notice that "hidden" parent categories are never in fact hidden on category pages (although they are listed separately).
Hidden categories are listed at the bottom when previewing. All users of the desktop version can see hidden categories for a page by clicking "Page information" under "Tools" in the left pane, or by editing the whole page with the source editor.
====Redirected categories====
Do not create inter-category redirects. See [[Wikipedia:Categories for discussion#Redirecting categories]] for the policy, and [[Wikipedia:Redirect#Category redirects]] for the technical details.
{{Anchor|Sort order}}
==Sort keys==
{{redirect|WP:Sorting|sorting of tables|Help:Sorting}}
{{shortcut|WP:SORTKEY}}
'''Sort keys''' are sometimes needed to produce a correct ordering of member pages and subcategories on the category page. For the mechanics, see [[Help:Category#Sorting category pages|Sorting category pages]] on the help page.
Categories of people are usually sorted by last name rather than first name, so "surname, forename" sort keys are used (as in "Washington, George"). There are many other rules for sorting people's names; for more information, see [[WP:NAMESORT]].
Other sort key considerations (in no particular order):
# In English Wikipedia, sort order merges (ignores) case and diacritics. For example, "Baé", "Båf", "BaG" would be sorted in that order.
# The main article/s of a category, if existent, should get sorted with a space as key so that it/they appear at the very top of the category. Example: <code><nowiki>[[Category:Example| ]]</nowiki></code> Those articles are typically homonymous or at least synonymous to their category.<!--generally shouldn't exceed 3 articles; most of the time it should be a single one--> Furthermore, other general articles that are highly relevant to the category should be sorted with an asterisk as key so that they also appear at the top of a category but beneath the main article/s. Example: <code><nowiki>[[Category:Example|*]]</nowiki></code> Those articles are typically called "History of example", "Types of example", "List of example" or similar.<!--generally shouldn't exceed 5 articles; most of the time it should be a single one-->
# [[Article (part of speech)|Leading articles]]—a, an, and the—are among the most common reasons for using sort keys, which are used to transfer the leading article to the end of the key, as in <nowiki>{{DEFAULTSORT:Lady, The}}</nowiki>. Please also apply these sort keys to deliberate misspellings of these words, ''e.g.'' "da" or "tha" for "the", as well as foreign language leading articles, such as "el" or "der" (but beware of non-article words that have the same spelling, e.g. that translate as "at" or "one"). However, leading articles in foreign-language-derived names which are no longer translated in English are not subject to this rule; e.g. the sort key for [[El Paso]] should be left as the default value (i.e. no <nowiki>{{DEFAULTSORT}}</nowiki> required).
# Spell out abbreviations and characters used in place of words so that they can be found easily in categories. E.g. the sort key for [[Mr. Bean]] should be <nowiki>{{DEFAULTSORT:Mister Bean}}</nowiki> and [[Dungeons & Dragons]] should be sorted <nowiki>{{DEFAULTSORT:Dungeons And Dragons}}</nowiki>. An exception is the times sign (×) as in "Men's 4 × 100 metre" relay; use the letter x in this case.
# Hyphens, apostrophes and periods/full stops are the only punctuation marks that should be kept in sort values. The only exception is the apostrophe in names beginning with ''O{{'}}'', which should be removed. For example, [[Eugene O'Neill]] is sorted <nowiki>{{DEFAULTSORT:Oneill, Eugene}}</nowiki>. All other punctuation marks should be removed.
# Entries containing numbers sometimes need special sort keys to ensure proper numerical ordering. For example, IX comes before V in alphabetical order, so [[Pope John IX]] might have a sort key "John 9". Also note that numbers which include separators (such as commas or periods) will only have the part of the number before the first separator considered by the sorting algorithm. So [[10,000 Maniacs]] might have a sort key "10000 Maniacs". It is important to stick to the same system for all similar entries in a given category.
# Systematic sort keys are also used in other categories where the logical sort order is not alphabetical (for example, individual month articles in year categories such as [[:Category:2004]] use sort keys like "*2004-04" for April). Again, such systems must be used consistently within a category.
# In some categories, sort keys are used to exclude prefixes that are common to all or many of the entries, or are considered unimportant (such as "List of" or "The"). For example, in [[:Category:2004]] the page [[2004 in film]] would have the sort key "Film", and in [[:Category:2004 in Canada]] the page [[2004 Canadian federal budget]] would have the sort key "Federal Budget".
# Use a space as the sort key for a [[Wikipedia:Glossary#Key article|key article]] for the category. (Note: If the key article should not be a member, simply edit the category text itself to add it, perhaps using {{tl|Cat main}}.)
# Use other sort keys beginning with a space (or an asterisk or a plus sign) for any "List of ..." and other pages that should appear ''after'' the key article and ''before'' the main alphabetical listings, including "Outline of" and "Index of" pages. The same technique is sometimes used to bring particular subcategories to the start of the list.
# To place entries ''after'' the main alphabetical list, use sort keys beginning with tilde ("~"). Several [[Greek alphabet|Greek letters]] are also used for specific purposes.
#* "[[Σ]]" (capital sigma) is used to place [[Wikipedia:Stub|stub]] categories at the end of subcategory lists. ("µ" ([[Mu (letter)|mu]]) was previously used, but the capital version "Μ" was confusing.)
#: (The remaining letters in this list are all lower case.)
#* "[[β]]" (beta, displays as capital, "Β") is for [[Wikipedia:Books|Wikipedia books]].
#* "[[ι]]" (iota, displays as "Ι") is for [[Wikipedia:Images|Wikipedia images]].
#* "[[ρ]]" (rho, displays as "Ρ") is for [[Wikipedia:Portal|portals]].
#* "[[τ]]" (tau, displays as "Τ") is for [[Help:Template|templates]].
#* "[[ω]]" (omega, displays as "Ω") is for [[WikiProject]]s.
#:Similar to the handling of Latin letters, if the sort key is or begins with a lower case Greek letter, then the capital Greek letter will be displayed in headings on category pages. Items whose sort keys begin with "β", "ι", "ρ", "Σ", "τ", or "ω" will appear beneath "[[Β]]" (capital beta), "[[Ι]]" (capital iota), "[[Ρ]]" (capital rho), "[[Σ]]" (capital sigma), "[[Τ]]" (capital tau), or "[[Ω]]" (capital omega) respectively. Several of these resemble Latin letters B, I, P etc., but they will sort after Z.
# If a page is to be given the same sort key in all or several of its categories, the <code><nowiki>{{DEFAULTSORT}}</nowiki></code> [[Help:Magic words|magic word]] should be used. Per [[WP:FOOTERS]], this is placed just before the list of category declarations. Default sort keys are sometimes defined even where they do not seem necessary—when they are the same as the page name, for example—in order to prevent other editors or automated tools from trying to infer a different default.
==Category tree organization==
[[Image:Category-diagram.png|thumb|300px|thumb|right|Partial view of Wikipedia's category system. Arrows point from category to sub-category.]]
Categories are organized as overlapping "[[tree (graph theory)|trees]]", formed by creating links between inter-related categories (in mathematics or computer science this structure is called <!---wp's cat.s don't have unique joins or meets---a lattice or---> a [[partially ordered set]]). Any category may contain (or "branch into") '''subcategories''', and it is possible for a category to be a subcategory of more than one "parent" category. (A is said to be a '''parent category''' of B when B is a subcategory of A.) <ref>Mathematically speaking, this means that the system approximates a [[directed acyclic graph]].</ref>
There is one top-level category, [[:Category:Contents]]. All other categories are found below this. Hence every category apart from this top one must be a subcategory of at least one other category.
{{Anchor|Topic category|Topic categories|Set category|Set categories}}
{{Shortcut|WP:TOPICCAT|WP:SETCAT}}
There are two main kinds of category:
* '''Topic categories''' are named after a topic (usually sharing a name with the Wikipedia article on that topic). For example, [[:Category:France]] contains articles relating to the topic [[France]].
* '''Set categories''' are named after a class (usually in the plural). For example, [[:Category:Cities in France]] contains articles whose subjects are cities in France.
Sometimes, for convenience, the two types can be combined, to create a ''set-and-topic'' category (such as [[:Category:Voivodeships of Poland]], which contains articles about particular voivodeships as well as articles relating to voivodeships in general).
=== Subcategorization ===
{{Shortcut|WP:SUBCAT|WP:SUPERCAT}}
[[File:Binary tree structure.svg|thumb|300px|A tree structure showing the possible hierarchical organization of an encyclopedia]]
[[File:Tree product pract.svg|thumb|Items may belong to more than one category, but normally not to a category and its parent (there are, however, exceptions to this rule, such as [[#Non-diffusing subcategories|non-diffusing categories]]). An item may belong to several subcategories of a parent category (as pictured).]]
If logical membership of one category implies logical membership of a second (an [[is-a]] relationship), then the first category should be made a subcategory (directly or indirectly) of the second. For example, [[:Category:Cities in France|Cities in France]] is a subcategory of [[:Category:Populated places in France|Populated places in France]], which in turn is a subcategory of [[:Category:Geography of France|Geography of France]].
Many subcategories have two or more parent categories. For example, [[:Category:British writers]] should be in both [[:Category:Writers by nationality]] and [[:Category:British people by occupation]]. When making one category a subcategory of another, ensure that the members of the subcategory really can be expected (with possibly a few exceptions) to belong to the parent also. Category chains formed by parent–child relationships should never form closed loops;<ref>This condition can be formulated in terms of [[graph theory]] as follows: the directed graph that has the categories as vertices and the parent-child relationships as edges should be ''[[directed acyclic graph|acyclic]]''.</ref> that is, no category should be contained as a subcategory of one of its own subcategories.{{#tag:ref|There is an exception to this for maintenance purposes. For example, [[:Category:Hidden categories]] is a direct subcategory of itself and of [[:Category:Wikipedia pages under 30-500 editing restriction]] and [[:Category:Container categories]], each of which is a direct subcategory of [[:Category:Hidden categories]].}} If two categories are closely related but are not in a subset relation, then links between them can be included in the text of the category pages.
Apart from certain exceptions (i.e. non-diffusing subcategories, see below), an article should be categorised as low down in the category hierarchy as possible, without duplication in parent categories above it. In other words, a page or category should rarely be placed in both a category and a subcategory or parent category (supercategory) of that category (unless the child category is non-diffusing – see below – or [[#Eponymous categories|eponymous]]). For example, the article "Paris" need only be placed in "Category:Cities in France", ''not'' in both "[[:Category:Cities in France]]" and "[[:Category:Populated places in France]]". Because the first category (cities) is in the second category (populated places), readers are already given the information that Paris is a populated place in France by it being a city in France.
Sometimes proper subcategorization requires the [[#Creating category pages|creation of new categories]].
Note also that as stub templates are for maintenance purposes, not user browsing (see [[#Wikipedia administrative categories]] above), they do ''not'' count as categorization for the purposes of Wikipedia's categorization policies. An article which has a "stubs" category on it ''must'' still be filed in the most appropriate content categories, even if one of them is a direct parent of the stubs category in question.
====Diffusing large categories====
{{Shortcut|WP:DIFFUSE}}
{{redirect|WP:DIFFUSE|conflicts|Wikipedia:Diffusing conflict}}
Although there is no limit on the size of categories, a large category will often be broken down ("diffused") into smaller, more specific subcategories. For example, [[:Category:Rivers of Europe]] is broken down by country into the subcategories [[:Category:Rivers of Albania|Rivers of Albania]], [[:Category:Rivers of Andorra|Rivers of Andorra]], etc.
{{anchor|Metacategories}}A category may be diffused using several coexisting schemes; for example, [[:Category:Albums]] is broken down by artist, by date, by genre etc. '''Metacategories''' may be created as ways of organizing schemes of subcategories. For example, the subcategories called "''Artistname'' albums" are not placed directly into [[:Category:Albums]], but into the metacategory [[:Category:Albums by artist]], which itself appears in Category:Albums. (see [[:Category:Categories by parameter]])
It is possible for a category to be only partially diffused—some members are placed in subcategories, while others remain in the main category.
Information about how a category is diffused may be given on the category page. Categories which are intended to be fully broken down into subcategories can be marked with the {{tl|category diffuse}} template, which indicates that any pages which editors might add to the main category should be moved to the appropriate subcategories when sufficient information is available. (If the proper subcategory for an article does not exist yet, either create the subcategory or leave the article in the parent category for the time being.)
To suggest that a category is so large that it ought to be diffused into subcategories, you can add the {{tl|overpopulated category}} template to the category page.
====Non-diffusing subcategories====
{{Shortcut|WP:DUPCAT}}
Not all subcategories serve the "diffusion" function described above; some are simply subsets which have some special characteristic of interest, such as [[:Category:Best Actor Academy Award winners|Best Actor Academy Award winners]] as a subcategory of [[:Category:Film actors|Film actors]] or [[:Category:Musical films|Musical films]] as a subcategory of [[:Category:Musicals|Musicals]]. These are called '''non-diffusing subcategories'''. They provide an exception to the general rule that pages are not placed in both a category and its subcategory: there is no need to take pages out of the parent category ''purely'' because of their membership of a non-diffusing subcategory. (Of course, if the pages also belong to other subcategories that do cause diffusion, then they will not appear in the parent category directly.)
Non-diffusing subcategories should be identified with a template on the category page:
* The {{Tl|Non-diffusing subcategory}} templates should be used for sub-categories that are non-diffusing, like {{cl|American novelists of Asian descent}}.
* The {{Tl|All included}} can be used for categories where all child articles of a certain type are in the parent, like {{cl|Presidents of the United States}} or {{cl|Mountains of Switzerland}}.
Subcategories defined by gender, ethnicity, religion, and sexuality should almost always be non-diffusing subcategories. The [[Wikipedia:Categorization/Ethnicity, gender, religion and sexuality]] categorization guideline outlines the rules on these categories in more detail.
Note that some categories can be non-diffusing on some parents, and diffusing on others. For example, {{cl|British women novelists}} is a non-diffusing sub-category of {{cl|British novelists}}, but it is a diffusing subcategory of {{cl|Women novelists by nationality}}.
==Category cleanup templates==
;Article with insufficient categories
:{{Tl|Improve categories}} template indicates that the article needs additional or more specific categories. It is recommended that this template be placed at the ''bottom'' of the page, where readers will look for the categories.
;Article with too many categories
:Use {{Tl|Recategorize}} template when there are too many categories. Put this template ''on the top'' of articles.
;Incorrect category
:The {{Tl|Check category}} template can be used to flag a page that may be in the wrong category or which you believe may need adding to additional categories. It will add the page to [[:Category:Category needs checking]].
;Category unknown
:If you're not sure where to categorise a particular page, add the {{Tl|uncategorized}} template to it, and other editors (such as those monitoring [[Wikipedia:WikiProject Categories/uncategorized]]) will help find appropriate categories for it.
==See also==
* [[Wikipedia:Categorization dos and don'ts]] (information page summarizing key points of this guideline)
* [[Wikipedia:Category suppression]]
* [[Wikipedia:WikiProject Categories]]
* [[m:Association of Categorist Wikipedians]]
* [[m:Help:Sorting]]
* [[:Category:Wikipedia essays about categorization]]
* [[Wikipedia:FAQ/Categorization]]
* [[Help:Gadget-Cat-a-lot]], script for moving subcategories between categories
===For browsing===
* [[Wikipedia:PetScan]] (formerly CatScan)
* [[Wikipedia:Classification]] (category tree jumping)
* [[Wikipedia:Category intersection]] (ability to find articles that are in more than one category)
* [[Special:Categories]] (lists all existing categories alphabetically)
* [[Special:CategoryTree]]
* [[:Category:Wikipedia categories]]
===For maintenance===
* [[Special:Mostlinkedcategories]]
* [[Special:Uncategorizedimages]]
* [[Special:Uncategorizedpages]]
* [[Special:Unusedcategories]]
* [[Special:Wantedcategories]]
* [[Wikipedia:Category combinatorics]]
* [[Wikipedia:Database reports#Categories]]
* [[Template:Category link with count]]
==Notes==
{{reflist}}
{{Wikipedia policies and guidelines}}
[[Category:Wikipedia categorization| ]]
[[Category:Wikipedia how-to]]
[[Category:Wikipedia project content guidelines]]
{{Hatnote|"WP:CATEGORY" and "WP:CAT" redirect here. You may be looking for [[Wikipedia:WikiProject Cats|WikiProject Cats]], [[Wikipedia:HotCat]], or [[Wikipedia:WikiProject Categories|WikiProject Categories]]. For the category system itself, see [[:Category:Contents]].}}
{{Subcat guideline|editing guideline|Categorization|WP:CAT|WP:CATEGORY|WP:CG|WP:MOSCAT}}
{{Guideline list}}
This page contains [[Wikipedia:Policies and guidelines |guidance]] on the proper use of the '''categorization''' function in Wikipedia. For information on the mechanics of the function, category syntax, etc., see '''[[Help:Category]]'''. For quick answers, see the '''[[Wikipedia:FAQ/Categorization|Categorization FAQ]]'''. For proposals to delete, merge, or rename categories, follow the instructions at [[Wikipedia:Categories for discussion|Categories for discussion]]. Please use it before undertaking any complicated re-categorization of existing categories or mass creation of new categories.
The central goal of the category system is to provide navigational links to Wikipedia pages in a hierarchy of ''categories'' which readers, knowing essential—''defining''—characteristics of a topic, can browse and quickly find sets of pages on topics that are defined by those characteristics.
Categories are not the only means of enabling users to browse sets of related pages. Other tools which may be used instead of or alongside categories in particular instances include lists and navigation boxes. For a comparison of these techniques, see [[Wikipedia:Categories, lists, and navigation templates|Categories, lists and navigation templates]].
== Naming conventions ==
{{Further|Wikipedia:Category names}}
{{Wikipedia:Categorization/Naming}}
==Creating category pages==
{{Also|Wikipedia:HotCat}}
After you have determined an appropriate category name ''and'' know its parent category, you are ready to create the new category. To create a category, first add an article to that category. Do this by editing the article page. At the bottom, but before the [[H:ILL|interwiki links]] (if any are present), add the name of the new category, (e.g.: {{code|[[Category:New category name]]}} ), and save your edit. The as-yet-undefined category name will now appear as a red link in the article's category list at the bottom of the page.
Next, to create the category, click on that red link, which brings you back into the editor. Adding this new category into the appropriate parent category is much the same as with an article: at the bottom, simply add the parent category (e.g.: {{code|[[Category:Parent category name]]}} ), which should usually be a [[hypernym]] of the sub-category.
Sometimes, a common-sense guess based on the title of the category isn't enough to figure out whether a page should be listed in the category. So, rather than leave the text of a category page empty (containing only parent category declarations), it is helpful – to both readers and editors – to include a description of the category, indicating what pages it should contain, how they should be subcategorized, and so on.
In such cases, the desired contents of the category should be described on the category page, similar to how the [[WP:LSC|list selection criteria]] are described in a stand-alone list. The category description should make direct statements about the criteria by which pages should be selected for inclusion in (or exclusion from) the category. This description, not the category's name, defines the proper content of the category. Do not leave future editors to guess about what or who should be included from the title of the category. Even if the selection criteria might seem obvious to you, an explicit standard is helpful to others, especially if they are less familiar with the subject.
The description can also contain links to other Wikipedia pages, in particular to other related categories which do not appear directly as subcategories or parent categories, and to relevant categories at [[Wikipedia:Wikimedia sister projects|sister projects]], such as [[Wikipedia:Commons|Commons]]. Another technique that can be used is described at [[Wikipedia:Classification]]. Like [[Wikipedia:Disambiguation|disambiguation]] pages, category pages should not contain either citations to [[Wikipedia:Identifying reliable sources|reliable sources]] or [[Wikipedia:External links|external links]].
Various templates have been developed to make it easier to produce category descriptions; see [[:Category:Category namespace templates|Category namespace templates]]. There are hatnote templates including {{tl|Cat main}} and {{tl|Category see also}}; others are listed at [[Wikipedia:Hatnote#Categories]].
A maximum of 200 category entries are displayed per screen. To make navigating large categories easier, a table of contents can be used on the category page. The following templates are some of the ways of doing this:
* {{Tl|Category TOC}} – adds a complete table of contents (Top, 0–9, A–Z)
* {{Tl|Large category TOC 2}} – adds a complete table of contents with five subdivisions for each letter (Aa Ae Aj Ao At)
* {{Tl|Large category TOC}} – adds a complete table of contents with twenty-six subdivisions for each letter (Aa ... Az)
Subcategories are split alphabetically along with the articles, which means that the initial screen of a split category may not include all its subcategories. To make all subcategories display on each screen, add a category tree to the text of the category page, as described at the help page under [[Help:Category#Displaying category trees and page counts|Displaying category trees and page counts]].
Category pages can have [[Help:Interlanguage links|interlanguage links]] in the "Languages" list in the left sidebar (in the default skin), linking to corresponding categories in other language Wikipedias. To edit these on [[Wikidata]], click on the "Edit links" link at the end of the languages list.
==Categorizing pages==
[[File:Category explanation.ogv|thumb|A screencast showing how to categorise pages and explaining the usage of [[WP:HOTCAT|HotCat]]]]
{{see also|Wikipedia:Categorization dos and don'ts|Wikipedia:Categorizing redirects}}
Every Wikipedia page should belong to at least one category. (However, there is no need to categorize [[Wikipedia:Talk page|talk pages]], [[Wikipedia:Redirect|redirects]], or [[Wikipedia:User page|user pages]], though these may be placed in categories where appropriate.) In addition, each categorized page should be placed in all of the ''most specific'' categories to which it logically belongs. This means that if a page belongs to a [[#Subcategorization|subcategory]] of C (or a subcategory of a subcategory of C, and so on) then it is ''not'' normally placed directly into C. For exceptions to this rule, see [[#Eponymous categories|Eponymous categories]] and [[#Non-diffusing subcategories|Non-diffusing subcategories]] below.
While it should typically be clear from the name of an existing category which pages it should contain, the text of the category page may sometimes provide additional information on potential category contents. One way to determine if suitable categories already exist for a particular page is to check the categories of pages concerning similar or related topics. Another way is to search existing category names as described [[Help:Searching|here]] (top of page). Since all categories form part of a hierarchy, do not add categories to pages as if they are [[Tag (metadata)|tags]].
===Articles===
{{see also|Wikipedia:Categorization of people}}
{{For|category types to avoid|Wikipedia:Overcategorization}}
{{anchor|Verifiable|Verifiability}}{{Shortcut|WP:CATV|WP:CATVER}}
Categorization of articles must be '''[[Wikipedia:Verifiability|verifiable]]'''. It should be clear from verifiable information in the article why it was placed in each of its categories. Use the {{tl|Category unsourced}} template if you find an article in a category that is not shown by sources to be appropriate or if the article gives no clear indication for inclusion in a category.
Categorization must also maintain a '''[[Wikipedia:NPOV|neutral point of view]]'''. Categorizations appear on article pages without annotations or referencing to justify or explain their addition; editors should be conscious of the need to maintain a neutral point of view when creating categories or adding them to articles. Categorizations should generally be uncontroversial; if the category's topic is likely to spark controversy, then a list article (which can be annotated and referenced) is probably more appropriate. For example, a politician (not convicted of any crime) should not be added to a category of notable criminals.
{{Anchor|Defining characteristics|Defining}}{{Shortcut|WP:CATDEF|WP:CATDEFINING}}<section begin=DefiningDef /><section begin=DefiningFull />A central concept used in categorising articles is that of the '''[[WP:DEFINING|defining]]''' characteristics of a subject of the article. A defining characteristic is one that [[WP:RS|reliable sources]] ''commonly'' and ''consistently'' define<ref>in prose, as opposed to a tabular or list form</ref> the subject as having—such as nationality or notable profession (in the case of people), type of location or region (in the case of places), etc.<section end=DefiningDef /> For example, here: "Caravaggio, an Italian artist of the Baroque movement ...", ''Italian'', ''artist'', and ''Baroque'' may all be considered to be defining characteristics of the subject ''Caravaggio''.<section end=DefiningFull />
Particular considerations for categorizing articles:
{{Shortcut|MOS:CATORDER}}
* By convention, category declarations are placed at the end of the wikitext, but before any [[Wikipedia:Stub|stub]] templates (which themselves transclude categories).
*{{Anchor|CATORDER}} [[WP:EPONYMOUS|Eponymous categories]] should appear first. Beyond that, the order in which categories are placed on a page is not governed by any single rule (for example, it does not need to be alphabetical, although partially alphabetical ordering can sometimes be helpful). Normally the most essential, significant categories appear first.
* An article should never be left with a non-existent ([[Wikipedia:REDLINK|redlinked]]) category on it. Either the category should be created, or else the link should be removed or changed to a category that does exist.
* Categorization should not be made by the ''type'' of an article. A biographical article about a specific person, for example, does not belong in [[:Category:Biography (genre)]].
* Articles on fictional subjects should not be categorized in a manner that confuses them with real subjects. {{tl|Category see also}} is useful for interlinking examples of real-world and fictional phenomena
==== Eponymous categories ====
{{Shortcut|WP:EPON|WP:EPONYMOUS}}
{{see also|Category:Eponymous categories|Wikipedia:Overcategorization#Eponymous categories for people}}
A category that covers exactly the same topic as an article is known as the '''eponymous category''' for that article (e.g. [[New York City]] and [[:Category:New York City]]; [[Mekong]] and [[:Category:Mekong River]]).
An eponymous category should have only the categories of its article that are relevant to the category's content. For example:
*Both [[New York City]] and [[:Category:New York City]] fit well in [[:Category:Cities in New York (state)]].
*The article [[New York City]] is in [[:Category:Populated places established in 1624]], but this category is not necessarily relevant to the content of [[:Category:New York City]], so it should not be used on the eponymous category.
'''Guidelines for articles with eponymous categories'''
*The article itself should be a member of the eponymous category and should be sorted with a space to appear at the start of the listing (see [[#Sort keys]]).
*The article should be listed as the main article of the category using the {{tl|cat main}} template.
*Articles with an eponymous category may be categorized in the broader categories that would be present if there were no eponymous category (e.g. the article [[France]] appears in both [[:Category:France]] and [[:Category:Western Europe]], even though the latter category is the parent of the former category). Editors should decide by consensus which solution makes most sense for a category tree. There are three options:
# Keep both the eponymous category and the main article in the parent category. This is used in [[:Category:Western Europe]] to allow that region's country articles to be navigated together.
# Keep just the child article. This is used in [[:Category:British Islands]], to prevent a loop.
# Keep just the eponymous category. This is used for [[:Category:Farmworkers]] in [[:Category:People by occupation]]. Such "''X'' by ''Y''" categories sometimes cover a limited navigational set, not a topic (see [[#Category tree organization]]), thus there is no logical article content.
If eponymous categories are categorized separately from their articles, it will be helpful to make links between the category page containing the articles and the category page containing the eponymous categories. The template {{Tl|Related category}} can be used for this. An example of this set-up is the linked categories [[:Category:American politicians]] and [[:Category:Wikipedia categories named after American politicians]].
==={{anchor|Images}}Files/images===
{{shortcut|WP:FILECAT}}
{{see|Wikipedia:Manual of Style/Images#Image description pages}}
Category tags can be added to [[Help:Files|file]]/image pages of files that have been uploaded to Wikipedia. When categorized, files are not included in the count of articles in the category, but are displayed in a separate section with a thumbnail and the name for each. A category can mix articles and images, or a separate file/image category can be created. A file category is typically a subcategory of the general category about the same subject, and a subcategory of the wider category for files, [[:Category:Wikipedia files]]. To categorize a new file when uploading, simply add the category tag to the [[Help:Edit summary#File upload summary|upload summary]].
Freely licensed files may also be uploaded to, and categorized on, [[WP:COMMONS|Wikimedia Commons]]. This can be done instead of, or in addition to, uploading and categorizing on Wikipedia. Most freely licensed files will eventually be copied or moved from Wikipedia to Commons, with a mirror page remaining on Wikipedia. (For an example of one such mirror page, see [[:File:Niederwald memorial 2.JPG|here]].) Categories should not be added to these Wikipedia mirror pages, because doing so creates a new Wikipedia page that is subject to [[WP:SD#F2|speedy deletion]]. Exceptions to this principle are made for mirror pages of images that are nominated as [[Wikipedia:Featured pictures|featured pictures]] and for those that appear on the Wikipedia [[Main Page]] in the [[:Template:Did you know|Did You Know?]] column.
Images that are used in Wikipedia that are [[Wikipedia:Non-free content|non-free or fair use]] should not appear as thumbnail images in categories. To prevent the thumbnail preview of images from appearing in a category, <nowiki>__NOGALLERY__</nowiki> should be added to the text of the category. In such cases, the file will still appear in the category, but the actual image preview will not.
==={{anchor|Non-article and maintenance categories|Maintenance categories}} Wikipedia administrative categories ===
{{Shortcut|WP:PROJCATS}}
{{see|Wikipedia:Administration#Data structure and development}}
A distinction is made between two types of categories:
* '''Administrative categories''', intended for use by editors or by automated tools, based on features of the current state of articles, or used to categorize non-article pages.
* '''Content categories''', intended as part of the encyclopedia, to help readers find articles, based on features of the subjects of those articles;
Administrative categories include '''stub categories''' (generally produced by [[Wikipedia:Stub|stub]] templates), '''maintenance categories''' (often produced by tag templates such as {{tl|cleanup}} and {{tl|fact}}, and used for [[Wikipedia:Maintenance|maintenance]] projects), '''[[Wikipedia:WikiProject|WikiProject]] and [[Wikipedia:Assessment|assessment categories]]''', and '''categories of pages in non-article [[Wikipedia:Namespace|namespaces]]'''.
Article pages should be kept out of administrative categories if possible. For example, the templates that generate WikiProject and assessment categories should be placed on talk pages, not on the articles themselves. If it is unavoidable that an administration category appears on article pages (usually because it is generated by a maintenance tag that is placed on articles), then in most cases it should be made a '''hidden category''', as described in [[#Hiding categories|§ Hiding categories]] below.
There are separate administrative categories for different kinds of non-article pages, such as [[Wikipedia:Template|template]] categories, [[Wikipedia:Disambiguation|disambiguation page]] categories, [[Wikipedia:Wikipedia namespace|project page]] categories etc.
==={{anchor|User namespace|Categorizing user pages}} User pages ===
{{shortcut|WP:USERNOCAT}}
{{Further|Wikipedia:User categories|Wikipedia:User pages#Categories, templates that add categories, and redirects}}
[[Wikipedia:User pages|User pages]] are not articles, and thus do not belong in ''content categories'' such as [[:Category:Living people|Living people]] or [[:Category:Biologists|Biologists]]. They can however be placed in '''user categories''' – subcategories of [[:Category:Wikipedians]], such as [[:Category:Wikipedian biologists]] – which assist collaboration between users.
Similarly, [[WP:SP|user subpages]] that are draft versions of articles should be kept out of content categories, but are permitted in non-content or project categories, like [[:Category:User essays]]. If you copy an article from mainspace to userspace and it already contains categories, remove them or [[Help:Hidden text|comment them out]]. Restore the categories when you move the draft back into article space.
At [[WP:Database reports/Polluted categories|Database reports/Polluted categories]], a list of affected categories is maintained.
=== Draft pages ===
{{shortcut|WP:DRAFTNOCAT}}
{{Further|Wikipedia:Drafts#Preparing drafts}}
Pages in the [[Wikipedia:Drafts|draft namespace]] are not articles, and thus do not belong in content categories such as [[:Category:Living people|Living people]] or [[:Category:Biologists|Biologists]]. <!-- They may be placed in ???? --> If you copy an article from mainspace to draftspace and it already contains categories, then disable them: edit each by inserting a colon character to [[Wikipedia:How to link to a category|link to its category]] instead (e.g. change <code><nowiki>[[Category:Biologists]]</nowiki></code> to <code><nowiki>[[:Category:Biologists]]</nowiki></code>), or [[Help:Hidden text|comment them out]] (e.g., <code><nowiki><!-- [[Category:Biologists]] --></nowiki></code>), or insert "nowiki" tags (e.g., {{tag|nowiki|content=<nowiki>[[Category:Biologists]]</nowiki>}}). Re-enable the categories when you move the draft back into article space.
==={{Anchor|T}}Template categorization===
{{shortcut|WP:CAT#T}}
[[Wikipedia:Template namespace|Templates]] should be categorized according to kind of template, but not by template content. For example, [[:Template:Schubert string quartets]] is categorized under [[:Category:String quartets by composer templates]], which should be a subcategory of [[:Category:Music navigational boxes]] (kind) but [[:Template:Schubert string quartets]] should ''not'' be categorized under [[:Category:Franz Schubert]] or [[:Category:String quartets]] (content).
It is usually desirable that pages using a template are not placed in the same categories as the template itself. To avoid this, the category for the template should be placed on the template's documentation page, normally within a {{tag|includeonly}} block; if there is no documentation page, the category for the template may be placed on the template itself, within a {{tag|noinclude}} block. Where a {{tag|noinclude}} block is the last item in the template code, there should be no spaces or newlines between the last part of the template proper and the opening {{tag|noinclude|o}} tag.
===Categorization using templates===
{{shortcut|WP:TEMPLATECAT|WP:TCAT}}
Many [[Wikipedia:Template messages|templates]] include category declarations in their transcludable text, for the purpose of [[:Category:Tracking categories|placing the pages containing those templates into specific categories]]. This technique is very commonly used for populating certain kinds of administration categories, including stub categories and maintenance categories.
However, it is recommended that articles not be placed in ordinary ''content'' categories using templates in this way. There are many reasons for this: editors cannot see the category in the wikitext; removing or restructuring the category is made more difficult (partly because automated processes will not work); inappropriate articles and non-article pages may get added to the category; sort keys may be unavailable to be customised per category; and ordering of categories on the page is less controllable.
When templates are used to populate administration categories, ensure that the code cannot generate nonsensical or non-existent categories, particularly when the category name depends on a parameter. Also, see [[Wikipedia:Category suppression|Category suppression]] for ways of keeping inappropriate pages out of template-generated categories.
Category declarations in templates often use <code><nowiki>{{PAGENAME}}</nowiki></code> as the sort key, because this overrides any DEFAULTSORT defined on the page.
====Hiding categories====
{{Shortcut|WP:HIDDENCAT}}
In cases where, for technical reasons, administration categories appear directly on articles rather than talk pages, they should be made into '''hidden categories''', so that they are not displayed to readers. This rule does not apply to [[Wikipedia:Stub|stub]] categories or "uncategorized article" categories – these types are ''not'' hidden.
To hide a category, add the template {{tlx|Wikipedia category|<nowiki>hidden=yes</nowiki>}} to the category page (the template uses the [[Wikipedia:Magic word|magic word]] <code><nowiki>__HIDDENCAT__</nowiki></code>). This also places the page in [[:Category:Hidden categories]].
A logged-in user may elect to view all hidden categories, by checking "Show hidden categories" on the "Appearance" tab of [[Special:Preferences|Preferences]]. Notice that "hidden" parent categories are never in fact hidden on category pages (although they are listed separately).
Hidden categories are listed at the bottom when previewing. All users of the desktop version can see hidden categories for a page by clicking "Page information" under "Tools" in the left pane, or by editing the whole page with the source editor.
====Redirected categories====
Do not create inter-category redirects. See [[Wikipedia:Categories for discussion#Redirecting categories]] for the policy, and [[Wikipedia:Redirect#Category redirects]] for the technical details.
{{Anchor|Sort order}}
==Sort keys==
{{redirect|WP:Sorting|sorting of tables|Help:Sorting}}
{{shortcut|WP:SORTKEY}}
'''Sort keys''' are sometimes needed to produce a correct ordering of member pages and subcategories on the category page. For the mechanics, see [[Help:Category#Sorting category pages|Sorting category pages]] on the help page.
Categories of people are usually sorted by last name rather than first name, so "surname, forename" sort keys are used (as in "Washington, George"). There are many other rules for sorting people's names; for more information, see [[WP:NAMESORT]].
Other sort key considerations (in no particular order):
# In English Wikipedia, sort order merges (ignores) case and diacritics. For example, "Baé", "Båf", "BaG" would be sorted in that order.
# The main article/s of a category, if existent, should get sorted with a space as key so that it/they appear at the very top of the category. Example: <code><nowiki>[[Category:Example| ]]</nowiki></code> Those articles are typically homonymous or at least synonymous to their category.<!--generally shouldn't exceed 3 articles; most of the time it should be a single one--> Furthermore, other general articles that are highly relevant to the category should be sorted with an asterisk as key so that they also appear at the top of a category but beneath the main article/s. Example: <code><nowiki>[[Category:Example|*]]</nowiki></code> Those articles are typically called "History of example", "Types of example", "List of example" or similar.<!--generally shouldn't exceed 5 articles; most of the time it should be a single one-->
# [[Article (part of speech)|Leading articles]]—a, an, and the—are among the most common reasons for using sort keys, which are used to transfer the leading article to the end of the key, as in <nowiki>{{DEFAULTSORT:Lady, The}}</nowiki>. Please also apply these sort keys to deliberate misspellings of these words, ''e.g.'' "da" or "tha" for "the", as well as foreign language leading articles, such as "el" or "der" (but beware of non-article words that have the same spelling, e.g. that translate as "at" or "one"). However, leading articles in foreign-language-derived names which are no longer translated in English are not subject to this rule; e.g. the sort key for [[El Paso]] should be left as the default value (i.e. no <nowiki>{{DEFAULTSORT}}</nowiki> required).
# Spell out abbreviations and characters used in place of words so that they can be found easily in categories. E.g. the sort key for [[Mr. Bean]] should be <nowiki>{{DEFAULTSORT:Mister Bean}}</nowiki> and [[Dungeons & Dragons]] should be sorted <nowiki>{{DEFAULTSORT:Dungeons And Dragons}}</nowiki>. An exception is the times sign (×) as in "Men's 4 × 100 metre" relay; use the letter x in this case.
# Hyphens, apostrophes and periods/full stops are the only punctuation marks that should be kept in sort values. The only exception is the apostrophe in names beginning with ''O{{'}}'', which should be removed. For example, [[Eugene O'Neill]] is sorted <nowiki>{{DEFAULTSORT:Oneill, Eugene}}</nowiki>. All other punctuation marks should be removed.
# Entries containing numbers sometimes need special sort keys to ensure proper numerical ordering. For example, IX comes before V in alphabetical order, so [[Pope John IX]] might have a sort key "John 9". Also note that numbers which include separators (such as commas or periods) will only have the part of the number before the first separator considered by the sorting algorithm. So [[10,000 Maniacs]] might have a sort key "10000 Maniacs". It is important to stick to the same system for all similar entries in a given category.
# Systematic sort keys are also used in other categories where the logical sort order is not alphabetical (for example, individual month articles in year categories such as [[:Category:2004]] use sort keys like "*2004-04" for April). Again, such systems must be used consistently within a category.
# In some categories, sort keys are used to exclude prefixes that are common to all or many of the entries, or are considered unimportant (such as "List of" or "The"). For example, in [[:Category:2004]] the page [[2004 in film]] would have the sort key "Film", and in [[:Category:2004 in Canada]] the page [[2004 Canadian federal budget]] would have the sort key "Federal Budget".
# Use a space as the sort key for a [[Wikipedia:Glossary#Key article|key article]] for the category. (Note: If the key article should not be a member, simply edit the category text itself to add it, perhaps using {{tl|Cat main}}.)
# Use other sort keys beginning with a space (or an asterisk or a plus sign) for any "List of ..." and other pages that should appear ''after'' the key article and ''before'' the main alphabetical listings, including "Outline of" and "Index of" pages. The same technique is sometimes used to bring particular subcategories to the start of the list.
# To place entries ''after'' the main alphabetical list, use sort keys beginning with tilde ("~"). Several [[Greek alphabet|Greek letters]] are also used for specific purposes.
#* "[[Σ]]" (capital sigma) is used to place [[Wikipedia:Stub|stub]] categories at the end of subcategory lists. ("µ" ([[Mu (letter)|mu]]) was previously used, but the capital version "Μ" was confusing.)
#: (The remaining letters in this list are all lower case.)
#* "[[β]]" (beta, displays as capital, "Β") is for [[Wikipedia:Books|Wikipedia books]].
#* "[[ι]]" (iota, displays as "Ι") is for [[Wikipedia:Images|Wikipedia images]].
#* "[[ρ]]" (rho, displays as "Ρ") is for [[Wikipedia:Portal|portals]].
#* "[[τ]]" (tau, displays as "Τ") is for [[Help:Template|templates]].
#* "[[ω]]" (omega, displays as "Ω") is for [[WikiProject]]s.
#:Similar to the handling of Latin letters, if the sort key is or begins with a lower case Greek letter, then the capital Greek letter will be displayed in headings on category pages. Items whose sort keys begin with "β", "ι", "ρ", "Σ", "τ", or "ω" will appear beneath "[[Β]]" (capital beta), "[[Ι]]" (capital iota), "[[Ρ]]" (capital rho), "[[Σ]]" (capital sigma), "[[Τ]]" (capital tau), or "[[Ω]]" (capital omega) respectively. Several of these resemble Latin letters B, I, P etc., but they will sort after Z.
# If a page is to be given the same sort key in all or several of its categories, the <code><nowiki>{{DEFAULTSORT}}</nowiki></code> [[Help:Magic words|magic word]] should be used. Per [[WP:FOOTERS]], this is placed just before the list of category declarations. Default sort keys are sometimes defined even where they do not seem necessary—when they are the same as the page name, for example—in order to prevent other editors or automated tools from trying to infer a different default.
==Category tree organization==
[[Image:Category-diagram.png|thumb|300px|thumb|right|Partial view of Wikipedia's category system. Arrows point from category to sub-category.]]
Categories are organized as overlapping "[[tree (graph theory)|trees]]", formed by creating links between inter-related categories (in mathematics or computer science this structure is called <!---wp's cat.s don't have unique joins or meets---a lattice or---> a [[partially ordered set]]). Any category may contain (or "branch into") '''subcategories''', and it is possible for a category to be a subcategory of more than one "parent" category. (A is said to be a '''parent category''' of B when B is a subcategory of A.) <ref>Mathematically speaking, this means that the system approximates a [[directed acyclic graph]].</ref>
There is one top-level category, [[:Category:Contents]]. All other categories are found below this. Hence every category apart from this top one must be a subcategory of at least one other category.
{{Anchor|Topic category|Topic categories|Set category|Set categories}}
{{Shortcut|WP:TOPICCAT|WP:SETCAT}}
There are two main kinds of category:
* '''Topic categories''' are named after a topic (usually sharing a name with the Wikipedia article on that topic). For example, [[:Category:France]] contains articles relating to the topic [[France]].
* '''Set categories''' are named after a class (usually in the plural). For example, [[:Category:Cities in France]] contains articles whose subjects are cities in France.
Sometimes, for convenience, the two types can be combined, to create a ''set-and-topic'' category (such as [[:Category:Voivodeships of Poland]], which contains articles about particular voivodeships as well as articles relating to voivodeships in general).
=== Subcategorization ===
{{Shortcut|WP:SUBCAT|WP:SUPERCAT}}
[[File:Binary tree structure.svg|thumb|300px|A tree structure showing the possible hierarchical organization of an encyclopedia]]
[[File:Tree product pract.svg|thumb|Items may belong to more than one category, but normally not to a category and its parent (there are, however, exceptions to this rule, such as [[#Non-diffusing subcategories|non-diffusing categories]]). An item may belong to several subcategories of a parent category (as pictured).]]
If logical membership of one category implies logical membership of a second (an [[is-a]] relationship), then the first category should be made a subcategory (directly or indirectly) of the second. For example, [[:Category:Cities in France|Cities in France]] is a subcategory of [[:Category:Populated places in France|Populated places in France]], which in turn is a subcategory of [[:Category:Geography of France|Geography of France]].
Many subcategories have two or more parent categories. For example, [[:Category:British writers]] should be in both [[:Category:Writers by nationality]] and [[:Category:British people by occupation]]. When making one category a subcategory of another, ensure that the members of the subcategory really can be expected (with possibly a few exceptions) to belong to the parent also. Category chains formed by parent–child relationships should never form closed loops;<ref>This condition can be formulated in terms of [[graph theory]] as follows: the directed graph that has the categories as vertices and the parent-child relationships as edges should be ''[[directed acyclic graph|acyclic]]''.</ref> that is, no category should be contained as a subcategory of one of its own subcategories.{{#tag:ref|There is an exception to this for maintenance purposes. For example, [[:Category:Hidden categories]] is a direct subcategory of itself and of [[:Category:Wikipedia pages under 30-500 editing restriction]] and [[:Category:Container categories]], each of which is a direct subcategory of [[:Category:Hidden categories]].}} If two categories are closely related but are not in a subset relation, then links between them can be included in the text of the category pages.
Apart from certain exceptions (i.e. non-diffusing subcategories, see below), an article should be categorised as low down in the category hierarchy as possible, without duplication in parent categories above it. In other words, a page or category should rarely be placed in both a category and a subcategory or parent category (supercategory) of that category (unless the child category is non-diffusing – see below – or [[#Eponymous categories|eponymous]]). For example, the article "Paris" need only be placed in "Category:Cities in France", ''not'' in both "[[:Category:Cities in France]]" and "[[:Category:Populated places in France]]". Because the first category (cities) is in the second category (populated places), readers are already given the information that Paris is a populated place in France by it being a city in France.
Sometimes proper subcategorization requires the [[#Creating category pages|creation of new categories]].
Note also that as stub templates are for maintenance purposes, not user browsing (see [[#Wikipedia administrative categories]] above), they do ''not'' count as categorization for the purposes of Wikipedia's categorization policies. An article which has a "stubs" category on it ''must'' still be filed in the most appropriate content categories, even if one of them is a direct parent of the stubs category in question.
====Diffusing large categories====
{{Shortcut|WP:DIFFUSE}}
{{redirect|WP:DIFFUSE|conflicts|Wikipedia:Diffusing conflict}}
Although there is no limit on the size of categories, a large category will often be broken down ("diffused") into smaller, more specific subcategories. For example, [[:Category:Rivers of Europe]] is broken down by country into the subcategories [[:Category:Rivers of Albania|Rivers of Albania]], [[:Category:Rivers of Andorra|Rivers of Andorra]], etc.
{{anchor|Metacategories}}A category may be diffused using several coexisting schemes; for example, [[:Category:Albums]] is broken down by artist, by date, by genre etc. '''Metacategories''' may be created as ways of organizing schemes of subcategories. For example, the subcategories called "''Artistname'' albums" are not placed directly into [[:Category:Albums]], but into the metacategory [[:Category:Albums by artist]], which itself appears in Category:Albums. (see [[:Category:Categories by parameter]])
It is possible for a category to be only partially diffused—some members are placed in subcategories, while others remain in the main category.
Information about how a category is diffused may be given on the category page. Categories which are intended to be fully broken down into subcategories can be marked with the {{tl|category diffuse}} template, which indicates that any pages which editors might add to the main category should be moved to the appropriate subcategories when sufficient information is available. (If the proper subcategory for an article does not exist yet, either create the subcategory or leave the article in the parent category for the time being.)
To suggest that a category is so large that it ought to be diffused into subcategories, you can add the {{tl|overpopulated category}} template to the category page.
====Non-diffusing subcategories====
{{Shortcut|WP:DUPCAT}}
Not all subcategories serve the "diffusion" function described above; some are simply subsets which have some special characteristic of interest, such as [[:Category:Best Actor Academy Award winners|Best Actor Academy Award winners]] as a subcategory of [[:Category:Film actors|Film actors]] or [[:Category:Musical films|Musical films]] as a subcategory of [[:Category:Musicals|Musicals]]. These are called '''non-diffusing subcategories'''. They provide an exception to the general rule that pages are not placed in both a category and its subcategory: there is no need to take pages out of the parent category ''purely'' because of their membership of a non-diffusing subcategory. (Of course, if the pages also belong to other subcategories that do cause diffusion, then they will not appear in the parent category directly.)
Non-diffusing subcategories should be identified with a template on the category page:
* The {{Tl|Non-diffusing subcategory}} templates should be used for sub-categories that are non-diffusing, like {{cl|American novelists of Asian descent}}.
* The {{Tl|All included}} can be used for categories where all child articles of a certain type are in the parent, like {{cl|Presidents of the United States}} or {{cl|Mountains of Switzerland}}.
Subcategories defined by gender, ethnicity, religion, and sexuality should almost always be non-diffusing subcategories. The [[Wikipedia:Categorization/Ethnicity, gender, religion and sexuality]] categorization guideline outlines the rules on these categories in more detail.
Note that some categories can be non-diffusing on some parents, and diffusing on others. For example, {{cl|British women novelists}} is a non-diffusing sub-category of {{cl|British novelists}}, but it is a diffusing subcategory of {{cl|Women novelists by nationality}}.
==Category cleanup templates==
;Article with insufficient categories
:{{Tl|Improve categories}} template indicates that the article needs additional or more specific categories. It is recommended that this template be placed at the ''bottom'' of the page, where readers will look for the categories.
;Article with too many categories
:Use {{Tl|Recategorize}} template when there are too many categories. Put this template ''on the top'' of articles.
;Incorrect category
:The {{Tl|Check category}} template can be used to flag a page that may be in the wrong category or which you believe may need adding to additional categories. It will add the page to [[:Category:Category needs checking]].
;Category unknown
:If you're not sure where to categorise a particular page, add the {{Tl|uncategorized}} template to it, and other editors (such as those monitoring [[Wikipedia:WikiProject Categories/uncategorized]]) will help find appropriate categories for it.
==See also==
* [[Wikipedia:Categorization dos and don'ts]] (information page summarizing key points of this guideline)
* [[Wikipedia:Category suppression]]
* [[Wikipedia:WikiProject Categories]]
* [[m:Association of Categorist Wikipedians]]
* [[m:Help:Sorting]]
* [[:Category:Wikipedia essays about categorization]]
* [[Wikipedia:FAQ/Categorization]]
* [[Help:Gadget-Cat-a-lot]], script for moving subcategories between categories
===For browsing===
* [[Wikipedia:PetScan]] (formerly CatScan)
* [[Wikipedia:Classification]] (category tree jumping)
* [[Wikipedia:Category intersection]] (ability to find articles that are in more than one category)
* [[Special:Categories]] (lists all existing categories alphabetically)
* [[Special:CategoryTree]]
* [[:Category:Wikipedia categories]]
===For maintenance===
* [[Special:Mostlinkedcategories]]
* [[Special:Uncategorizedimages]]
* [[Special:Uncategorizedpages]]
* [[Special:Unusedcategories]]
* [[Special:Wantedcategories]]
* [[Wikipedia:Category combinatorics]]
* [[Wikipedia:Database reports#Categories]]
* [[Template:Category link with count]]
==Notes==
{{reflist}}
{{Wikipedia policies and guidelines}}
[[Category:Wikipedia categorization| ]]
[[Category:Wikipedia how-to]]
[[Category:Wikipedia project content guidelines]]
