Wikipedia:Categorization

When naming a category, be particularly careful to choose an accurate and neutral name. Moving non-conventionally categorized pages to another category name (see {{Category redirect}}) imposes an additional overhead – an edit for each article and subcategory. Also, try to avoid names that are too long or too short. Short, straight-forward names are preferred for categories, though it may be necessary at times to deviate from this principle for clarity or disambiguation.

• Names of topic categories should be singular, normally corresponding to the name of a Wikipedia article. Examples: "Law", "France", "Hillary Clinton".
• Names of set categories should be plural. Examples: "Writers", "Villages in Poland".
• Note that in many instances a topic category and a set category have similar names, the topic category being singular and the set category plural. For example, Opera is a topic category (containing all articles relating to the topic), while Operas is a set category (containing articles about specific operas). Be careful to choose the right one when categorizing articles.
• Do not write the category structure in names. Example: "Monarchs", not "People – Monarchs". This includes creating categories that are subpages of other categories.
• Choose category names that can stand alone, independent of the way a category is connected to other categories. Example: "Geography terminology stubs", not "Terminology" (a subcategory of "Geography stubs").

• Standard article naming conventions apply; in particular, use sentence case, do not use title case. Specifically, do not capitalize regular nouns except when they come at the beginning of the category name.
• When naming, use words and phrases which exist in verifiable reliable sources (particularly for technical subjects), so that those sources may be used to support inclusion of information.
• Avoid abbreviations. Example: "Category:Military equipment of World War II", not "Category:Military equipment of WW2". However, acronyms that have become the official, or generally used, name (such as NATO) should be used where there are no other conflicts. Exceptions to this should be explained in the category description.
• Avoid subjective adjectives such as famous, large, or beautiful in category titles.
• When a topic requires disambiguation, any category eponymously named for that topic should include the same form of disambiguation, even if no other articles are likely to have an eponymous category.
• If there are more than a few lists in a particular subject area, it may be appropriate to create a specific subcategory that contains only the list pages and no other types.
  • As an example Category:Countries contains the subcategory "Category:Lists of countries", which in turn contains the list pages "List of Soviet Republics", and "List of countries by population".
  • When placing list pages in such a category, use a sort key to adjust the page title to exclude the starting words "List of", or all lists would appear under "L". For example, when adding List of Soviet Republics to the category, using [[Category:Lists of countries|Soviet Republics]] and will make it be sorted in the S section. See also Category:Lists.
• Naming guidelines for subcategories of Category:Stub categories are listed at Wikipedia:WikiProject Stub sorting/Naming conventions § Stub categories.

• Categories used for Wikipedia administration, are disambiguated by prepending with the word "Wikipedia" (no colon) if this is needed to prevent confusion with content categories. For example, Category:Inactive project pages (where no confusion is likely), but Category:Wikipedia tools (as distinct from the content category Tools).
• All WikiProject categories should have "WikiProject" (or "WikiProjects") as part of the name. Note that this does not apply to the names of categories that projects might create for the use of assessments. In these cases, the advice given in Project Categories, which conforms to Category:Wikipedia 1.0 assessments is advised, though not mandatory.
• For geographical photo requests, the category name should be "Wikipedia requested photographs in xxx" as in [[Category:Wikipedia requested photographs in England]]; see Category:Wikipedia requested photographs by location.
• For categorization in userspace, see Wikipedia:Categorization § User pages and Wikipedia:User categories § Naming conventions.

Categories are not the only means of enabling users to browse sets of related pages. And categories have several disadvantages, including that categories only show the page name of each page being categorized, therefore:

• While the category description may provide broader information, individual category members cannot be annotated with descriptions or comments, so they give no context or elaboration for any specific entry.
• There is no provision for referencing any specific entry, to verify a page meets a category's criteria of inclusion.

So, consider whether a list would be more appropriate for such a grouping of pages. Lists would also be more useful where it is important to see dates, for example, a list of Nobel laureates. And can provide options for sorting the entries.

Also, consider whether the category might be considered category clutter. Please see Wikipedia:Overcategorization, for more information on this.

A good category name is neutral, and defines the category's inclusion criteria in the name itself, as much as possible.

Important: When choosing a name for the category, please be sure a similar category does not exist.

For example: You might want to add a subcategory for people to Category:Mexicans. Before creating that category, try to find it under a similar name. By starting at Category:People by nationality, you will discover that Mexicans are placed in Category:Mexican people.

Note: Eponymous categories (categories whose name and topic is the same as an article, such as a category named after a person) should only be created if enough directly related articles exist.

After you have determined an appropriate category name for the category, next try to find a suitable place for the new category. (For example, categories of people should be in the tree of "people" categories.) Please see #Category tree organization for more information on this.

Once you have determined where to categorize this new category, you should be ready to create the new category. To create a category,

1. Add a page to the intended category. Do this by editing that page:
   1. At the bottom, add the name of the new category, (e.g.: [[Category:New category name]] )
   2. Save your edit. The as-yet-undefined category name will now appear as a red link in the category list at the bottom of the page.
2. Next, click on that red link, which brings you to the new category page to create.
3. Finally, at the bottom of the page, simply add the parent category (e.g.: [[Category:Parent category name]] ), which should usually be a hypernym of the subcategory. This will add the new category into the appropriate parent category.

 * If something goes wrong, double check to see if you followed the steps properly and check if the wikitext is correct. For example, if the category fails to list in the parent category, the wikitext should be [[Category:Parent category name]], not [[:Category:Parent category name]].

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), adding a main article which describes the topic, can help with that.

However, only linking to a Wikipedia article explaining the title is often not sufficient as a description for a category. It can be helpful – to both readers and editors – to include a clear description of the category, indicating what pages it should contain, how they should be subcategorized, and so on, and linking to one or more pages as background information.

In such cases, the desired contents of the category should be described on the category page, similar to how the 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 sister projects, such as Commons. Another technique that can be used is described at Wikipedia:Classification. Like disambiguation pages, category pages should not contain either citations to reliable sources or external links.

Various templates have been developed to make it easier to produce category descriptions; see Category namespace templates. There are hatnote templates including {{Category main article}} and {{Category see also}}; others are listed at Wikipedia:Hatnote#Categories.

Categories may have hundreds of members, displayed over many pages, with, at most, only 200 category entries on a single category page. 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:

• {{Category TOC}} – adds a complete table of contents (Top, 0–9, A–Z)
• {{Large category TOC}} – adds a complete table of contents with five subdivisions for each letter (Aa Ae Aj Ao At)
• Consider using {{CatAutoTOC}} which uses the appropriate TOC for the number of pages on a category page.

Likewise, a maximum of 200 subcategories are displayed at a time, so some subcategories may not be immediately visible. To display all subcategories at once, add a category tree to the text of the category page, as described at Help:Category § Displaying category trees and page counts.

Category pages can have 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.

Every Wikipedia page should belong to at least one category, except for talk pages, redirects, and user pages, which may optionally be placed in categories where appropriate. 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 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 and § 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 here (top of page).

Since all categories form part of a tree-like hierarchy, do not add categories to pages as if they are tags.

Apart from certain exceptions (i.e. eponymous categories and non-diffusing subcategories – see below), an article should be categorised under the most specific branch in the category tree possible, without duplication in parent categories above it. In other words, articles should rarely be placed in both a given category and any of its sub- or parent (super-) categories. For example, the article "Paris" need only be placed in Category:Cities in France, not also in 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.

Categorization of articles must be verifiable. It should be clear from verifiable information in the article why it was placed in each of its categories. Use the {{Unreferenced category}} 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 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.

The defining characteristics of an article's topic are central to categorizing the article. A defining characteristic is one that reliable sources commonly and consistently refer to^[1] in describing the topic, such as the nationality of a person or the geographic location of a place. For example, Italian and artist are defining characteristics of Caravaggio, because virtually all reliable sources on the topic mention them, so that article is included in categories such as Category:Italian Baroque painters.

Be sure to include categories for all defining characteristics. For non-defining characteristics, editors should use their judgment to choose which additional categories (if any) to include. For example, Caravaggio is included in the non-defining category of Category:Deaths from sepsis.

Particular considerations for categorizing articles:

• By convention, category declarations are placed at the end of the wikitext, but before any stub templates, which transclude their own categories.
• 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 (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. {{Category see also}} is useful for interlinking examples of real-world and fictional phenomena.

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; Abraham Lincoln and Category:Abraham Lincoln).

Eponymous categories should not be created unless enough directly related articles or subcategories exist. However, this should not be done simply to reduce the number of categories displayed in an article.

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.

Individual works by a person should not be included in an eponymous category but should instead be in a subcategory such as Category:Novels by Agatha Christie.

• The article itself should be a member of the eponymous category. It should be sorted with a space, so that it appears at the start of the category listing (see § Sort keys below).
• The article should be listed as the main article of the category using the {{Category main article}} 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:Countries in 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:

1. Keep both the eponymous category and the main article in the parent category. This is used in Category:Countries in Europe to allow that region's country articles to be navigated together.
2. Keep just the child article. This is used in Category:British Islands, to prevent a loop.
3. Keep just the eponymous category. This is used for Category:Farmers in Category:People by occupation. Such "X by Y" categories sometimes cover a limited navigational set, not a topic (see § Category tree organization below), 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 {{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.

Category tags should be added to file 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 upload summary.

Freely licensed files should be uploaded to, and categorized on, Wikimedia Commons, instead of uploading and categorizing on Wikipedia. Existing freely licensed files should usually be moved from Wikipedia to Commons, with a mirror page automatically remaining on Wikipedia. (For an example of one such mirror page, see here.) Categories should not be added to these Wikipedia mirror pages, because doing so creates a new Wikipedia page that is subject to speedy deletion. Exceptions to this principle are made for mirror pages of images that are nominated as featured pictures and for those that appear on the Wikipedia Main Page in the Did You Know? column.

Images that are used in Wikipedia that are 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, __NOGALLERY__ 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.

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 stub templates), maintenance categories (often produced by tag templates such as {{cleanup}} and {{fact}}, and used for maintenance projects), WikiProject and assessment categories, and categories of pages in non-article 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 below.

There are separate administrative categories for different kinds of non-article pages, such as template categories, disambiguation page categories, project page categories etc.

In maintenance categories and other administrative categories, pages may be included regardless of type. For example, in an error tracking category it makes sense to group templates separately, because addressing the errors there may require different skills compared to fixing an ordinary article. For sorting each namespace separately, see § Sort keys below.

User pages are not articles, and thus do not belong in content categories such as Living people or Biologists. They can however be placed in user categories – subcategories of Category:Wikipedians, such as Category:Wikipedian biologists – which assist collaboration between users.

Similarly, 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, wrap them inside {{Draft categories}}, remove them, or comment them out. Restore the categories when you move the draft back into article space. Two scripts are available to help with these tasks: User:DannyS712/Draft no cat and User:DannyS712/Draft re cat.

Also, do not transclude articles into your user pages: this will result in the user page being included in all the article's categories.

At Database reports/Polluted categories, a list of affected categories is maintained.

Drafts, no matter whether in the draft namespace or your userspace, are not articles, and thus do not belong in content categories such as Living people or Biologists. If you copy an article from mainspace to draftspace or your userspace and it already contains categories, then disable those categories. This can be done either by inserting a colon character to link to each category (e.g. change [[Category:Biologists]] to [[:Category:Biologists]]), or by wrapping them in {{Draft categories}} (e.g.{{draft categories|1=[[Category:Living people]] [[Category:Biologists]]}} etc.). After you move the draft into article space, remove the leading colons to re-enable the categories. If using the draft categories template, the categories will automatically work as normal in mainspace, but the template should be removed. The same system may be used in a new draft to list the categories it may have when moved to mainspace.

Two scripts are available to help with these tasks: User:DannyS712/Draft no cat and User:DannyS712/Draft re cat.

Drafts may be placed in the appropriate subcategories of Category:Wikipedia drafts.

Templates are not articles, and thus do not belong in content categories. It is however a recommendation to place them in template categories – subcategories of Category:Wikipedia templates – to assist when looking for templates of a certain type. For example, Template:Schubert string quartets is categorized under Category:String quartets by composer navigational boxes, which should be a subcategory of Category:Music navigational boxes (type) 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 <includeonly>{{Sandbox other|...}}</includeonly> block; if there is no documentation page, the category for the template may be placed on the template itself, within a <noinclude>...</noinclude> block. When a <noinclude>...</noinclude> block is the last item in the template code, there should be no spaces or new lines between the last part of the template proper and the opening <noinclude> tag.

Many templates include category declarations in their transcludable text, for the purpose of 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. See Template:Infobox roller coaster for an example that only adds a category by manufacturer if it exists, and otherwise uses a hidden category.

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 Category suppression for ways of keeping inappropriate pages out of template-generated categories.

Category declarations in templates often use {{PAGENAME}} as the sort key, because this overrides any DEFAULTSORT defined on the page.

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 stub categories or "uncategorized article" categories – these types are not hidden.

To hide a category, add the template {{Wikipedia category|hidden=yes}} to the category page (the template uses the magic word __HIDDENCAT__). 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 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.

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.

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.

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.^[2]
• The main article/s of a category, if existent, should get sorted with a space as key so that it/they appear(s) at the very top of the category. Example: [[Category:Example| ]] Those articles are typically homonymous or at least synonymous to their category. 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: [[Category:Example|*]] Those articles are typically called "History of example", "Types of example", "List of example" or similar.
• 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 {{DEFAULTSORT:Lady, The}}. 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 {{DEFAULTSORT}} required).
• Spell out abbreviations and characters used in place of words so that they can be found easily in categories. For example, the sort key for Mr. Bean should be {{DEFAULTSORT:Mister Bean}} and Dungeons & Dragons should be sorted {{DEFAULTSORT:Dungeons And Dragons}}. An exception is the times sign (×) as in "Men's 4 × 100 metre" relay; use the letter x in this case.
• Landforms (and similar) that have noun prefixes such as Isle of Mull should have the noun sorted after as {{DEFAULTSORT:Mull, Isle of}}. However, this isn't usually done for settlements and administrative divisions; for example, while Isle of Wight uses {{DEFAULTSORT:Wight, Isle of}}, the categories for the county/district are sorted "Isle of Wight". Also for settlements such as Isle of Wight, Virginia the prefix isn't moved.
• 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 {{DEFAULTSORT:Oneill, Eugene}}. All other punctuation marks should be removed. (Commas can be added when re-ordering words, as in the previous example.)
• 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". To get the correct sort order zero padding may be required, thus the actual sort key in this case is "John 09" this ensures that Pope John IX sorts before Pope John X – if we ever get to the hundredth Pope John, we would need to use three digits "John 009". 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 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.

Sort order of characters before numbers and Latin alphabet (0–9, A–Z) is (partial list):

! " # $ % & ' ( ) * + , - . / 0 9 : ; < = > ? @ [ \ ] ^ _ ` A Z a z { | } ~ É é —

See also: Meta:Help:Sorting#Sort modes for more information.

• Sort keys may be prefixed with Greek letters to place entries after the main alphabetical list. The following letters have special meaning by convention:
  • "Σ" (capital sigma) is used to place stub categories at the end of subcategory lists. ("µ" (mu) was previously used, but the capital version "Μ" was confusing.)
  • "β" (beta, displays as capital, "Β") is for barnstars.
  • "Δ" (delta) is for documentation, where sorting by Latin D is undesirable.
  • "ι" (iota, displays as "Ι") is for Wikipedia images.
  • "ρ" (rho, displays as "Ρ") is for portals.
  • "τ" (tau, displays as "Τ") is for templates. Keep in mind, template categories should not be added to content categories per WP:CAT#T.
  • "υ" (upsilon, displays as "Υ") for user templates.
  • "ω" (omega, displays as "Ω") is for WikiProjects.

  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 lowercase letters will appear beneath corresponding capital letters. Several of these resemble Latin letters B, I, P etc., but they will sort after Z.

  Note: Not all of these types are suitable for inclusion in content categories. For one-type categories, such as template categories, Greek letter grouping is not useful.

• If a page is to be given the same sort key in all or several of its categories, the {{DEFAULTSORT}} magic word should be used. Per MOS:ORDER, 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.

 

Categories are organized as overlapping "trees", formed by creating links between inter-related categories (in mathematics or computer science this structure is called 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.) ^[3]

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.

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. A category may be explicitly labeled as such using the {{Set category}} template.

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).

 

 

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, Cities in France is a subcategory of Populated places in France, which in turn is a subcategory of 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;^[4] that is, no category should be contained as a subcategory of one of its own subcategories.^[5] 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.

Except for non-diffusing subcategories (see below), pages for subcategories should be categorised under the most specific parent categories possible.

Sometimes proper subcategorization requires the 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.

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 contains no articles about specific rivers directly, they are all in subcategories.

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 {{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, or substantially diffused, into subcategories, you can add the {{overpopulated category}} template to the category page.

Not all subcategories serve the "diffusion" function described above; some are simply subsets which have some special characteristic of interest, such as Best Actor Academy Award winners as a subcategory of Film actors. 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 {{Non-diffusing subcategory}} templates should be used for subcategories that are non-diffusing, like Category:American novelists of Asian descent.
• The {{All included}} can be used for categories where all child articles of a certain type are in the parent, like Category:Presidents of the United States or Category:Mountains of Switzerland.

Subcategories defined by gender, ethnicity, religion, and sexuality should almost always be non-diffusing subcategories. The Wikipedia:Categorizing articles about people 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, Category:British women novelists is a non-diffusing subcategory of Category:British novelists, but it is a diffusing subcategory of Category:Women novelists by nationality.

• Article with insufficient categories
  • {{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 {{Recategorize}} template when there are too many categories. Put this template on the top of articles.
• Category unknown
  • If you're not sure where to categorize a particular page, add the {{uncategorized}} template to it, and other editors (such as those monitoring Wikipedia:WikiProject Categories/uncategorized) will help find appropriate categories for it.

Anyone may edit an article and remove a questionable categorization. If an article has an "incorrect" or "inappropriate" category, remove that category from the article, and replace it (if applicable) with a more correct category.

Even if an article may occupy the grey areas of a category's inclusion criteria, that is not a valid reason to keep the article in a category. If a particular article does not fit the inclusion criteria of a category, then the article simply should not be added to it.

If categorization of any particular page is disputed, please discuss the categorization on the talk page of the page in question. If the category seems reasonable, but questionable in some cases, consider whether you can solve (part of) the problem by writing a clearer #Category description.

If you have a proposal for a better name for the category or for a wider re-arrangement of the categorization scheme; or if you have a concern that may apply to several members of the category (such as if the category violates one or more sections on this page, Wikipedia:Categorizing articles about people, Wikipedia:Overcategorization, or other Wikipedia policies and guidelines), you can participate in, or post a new, discussion about the category.

Category talk pages are not always widely watched. Consider whether you can invite more potentially interested people to take part in a discussion, such as by discussing it at a relevant WikiProject, or at Wikipedia:WikiProject Categories. Another option could be to nominate the category for discussion at Wikipedia:Categories for discussion, or if the category name has an obvious typographical error, you can list it for speedy renaming at Wikipedia:Categories for discussion/Speedy.

See Wikipedia:Categories for discussion/Howto for instructions on how to use the templates for: deletion ({{cfd}}), renaming ({{cfr}}), or merging ({{cfm}}).

If you are in a content dispute, see Wikipedia:Dispute resolution for what to do next.

• 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
• Wikipedia:Maintenance

• 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

• Special:MostLinkedCategories
• Special:UncategorizedFiles
• Special:UncategorizedPages
• Special:UnusedCategories
• Special:WantedCategories
• Wikipedia:Category combinatorics
• Wikipedia:Database reports#Categories
• Template:Category link with count
• Template:Wikipedia categorization navbox