The primary purpose of the categorization system on Wikipedia is to provide navigational links within a structured, tree-like hierarchy of categories. By grouping pages according to their essential, defining characteristics, the system allows readers to browse and efficiently locate related topics.
Proposals to delete, merge, or rename categories should follow the process outlined at Wikipedia:Categories for discussion. Please use that forum before undertaking extensive re-categorization of existing categories or the mass creation of new ones.
Categorizing pages
A screencast showing how to categorize pages and explaining the usage of the toolHotCat
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 only in the most specific categories to which it logically belongs, and subcategories should be categorized under only the most specificparent categories possible.
Important: All categories form part of a tree-like hierarchy. Do not add categories to pages as if they are tags.
Categories are not the only means of allowing users to browse sets of related pages, and have several disadvantages, including:
Categories only show the name of each page being categorized. While the category description may provide information about the category in general, 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 providing references for any given page in a category, making it difficult to verify that a page meets the category's criteria of inclusion.
So, consider whether a list would be more appropriate than a category (or in addition to a category) for a grouping of pages. As examples, a list of Nobel laureates could include dates, and a list formatted as a table can provide options for sorting the entries.
A good category name is accurate and neutral, and, as much as possible, defines the category's inclusion criteria in the name itself.
Important: Moving non-conventionally categorized pages to another category name (see {{Category redirect}}) imposes an additional overhead–an edit for every categorized page and subcategory. So, when choosing a name for the category, please be sure a category with a similar name or similar inclusion criteria does not already exist. (See also: WP:OVERLAPCAT.)
One way to determine if suitable categories already exist for a particular page is to check the categories on pages concerning similar or related topics. You can also search existing category names as described at Help:Searching (top of page).
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.
Creating the category
After you have determined an appropriate name for the category, find a suitable place for the new category. (For example, categories of people should be in the tree of "people" categories.) 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:
Add a page to the intended category. Do this by editing that page, and add the name of the new category: (e.g. [[Category:New category name]])
Per MOS:ORDER, categories 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.
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.
A page should never be left with a non-existent (red-linked) category on it. Either the category should be created, or else the link should be removed or changed to a category that does exist.
Next, click on that red link, which brings you to the new category page to create.
Finally, at the bottom of the category page, simply add a 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. (A category can have more than one parent category.)
Important: 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]].
While it should typically be clear from the name of an existing category which pages it should contain, sometimes, a common-sense guess based on the name of the category is not enough to figure out whether a page should be listed in the category. So, rather than leave the text of a category page empty (other than parent categories), adding a hatnote linking to the main article or another page which describes the topic (see §Articles with eponymous categories) 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 that do not appear directly as subcategories or parent categories, and to relevant categories at sister projects, such as Commons.
Categories may have hundreds or thousands of members, displayed over many pages, with up to 200 entries on a single 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 {{Automatic category TOC}}, which automatically generates the appropriate TOC for the number of pages in a category.
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 to corresponding categories in other language Wikipedias. Edit these on Wikidata by following the instructions at Help:Interlanguage links.
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.
Neutral: Categories appear on article pages without annotations or direct citations 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 with no formal conviction of any crime should not be added to a category for criminals.
Defining: Defining characteristics of an article's topic are central to categorizing the article. A defining characteristic is one that reliable sourcescommonly and consistently refer to in describing the topic, such as the nationality of a person or the geographic location of a place.
Articles should normally be included in the categories that are most closely related to the subject's defining characteristics.
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.
Categories should not group subjects by trivial characteristics that have little relevance to the topics, unless it can be shown that such a characteristic or grouping is notable.
For example, Category:Italian people with dark eyebrows would likely be trivia, even if it's true, so the category does not exist, and Caravaggio is not in it.
For the full category naming guidelines (including topic-specific naming conventions), see Wikipedia:Category names.
Standard article naming conventions apply: Generally, category names are based on what the subject is called in verifiablereliable sources (particularly for technical subjects), so that those sources may be used to support the inclusion of information. When this offers multiple possibilities, editors choose among them by considering several principles: the ideal category name uses words and phrases which precisely identify the subject; it is neutral, short, natural, distinguishable and recognizable; and resembles names for similar categories.
Avoid abbreviations. For 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.
Note that in some instances a topic category and a set category may have similar names–the topic category name is singular and the set category name is plural. Be careful to choose the right one when categorizing articles.
For example, Category:Opera is a topic category (containing all articles relating to the topic), while Category:Operas is a set category (containing articles about specific operas).
Sometimes, for convenience, the two types can be combined to create a set-and-topic category.
For example, Category:Voivodeships of Poland contains articles about particular voivodeships as well as articles relating to voivodeships in general.
For articles about people, categorize by characteristics of the person the article is about, not characteristics of the article: e.g. do not add Category:Biography. Subcategories of Category:Biography (genre) may legitimately contain articles about biographical films or biographical books, but should not contain articles about individual people. The article is a biography; the topic of the article–the person–is not.
Keep articles about people separate. Categories with a title indicating that the contents are people should normally only contain biographical articles and lists of people, and perhaps a non-biographical main article, though this can instead be linked in the category description. This is for clarity and ease of use, and to preserve the integrity of the category tree of people articles.
Editors should take particular care when categorizing articles about living people. All quotations and any material challenged or likely to be challenged must be supported by an inline citation to a reliable, published source. Contentious material about living (or, in some cases, recently deceased) persons that is unsourced or poorly sourced—whether the material is negative, positive, neutral, or just questionable—must be removed immediately and without waiting for discussion.
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.
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.
When placing list pages in a lists category, use a sort key to adjust the page title to exclude the starting words "List of", otherwise all lists would appear under "L".
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:
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 linked on the category page 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:
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.
Keep just the eponymous category in the parent 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.
Drafts, no matter whether in the draft namespace or your userspace, are not articles and thus do not belong in content categories such as Category:Living people or Category:Biologists. If you copy an article from mainspace to draftspace or userspace and it already contains categories, disable those categories. This can be done in any of the following four ways (but only the first two ways are recommended, as a disadvantage of the last two ways is that the categories will not be linked on the page):
Inserting a colon character to link to each category (e.g. changing [[Category:Biologists]] to [[:Category:Biologists]]).
Commenting (<!--...-->) them out (e.g. <!--[[Category:Living people]] [[Category:Biologists]]-->).
Wrapping them in <nowiki>...</nowiki> tags (e.g. <nowiki>[[Category:Living people]] [[Category:Biologists]]</nowiki>).
The same system may be used in a new draft to list the categories it may have when moved to mainspace. After you move the draft into article space, remove the leading colons, uncomment out the categories or remove the <nowiki>...</nowiki> tags to re-enable the categories. If you use the draft categories template, the categories will automatically work as normal in mainspace, but the template should be removed.
A distinction is made between two types of categories:
Administrative categories–intended for use by editors or 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.
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 administrative category appears on article pages (usually because it is generated by a maintenance template that is placed on articles), then in most cases the category should be made a hidden category, as described in §Hiding categories below.
Maintenance categories should never be added to articles directly. Instead, the categories should be added using templates.
In some administrative categories, pages may be included regardless of type or namespace. 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.
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 image preview will not.
Pages using a template should rarely be placed in the same categories as the template itself. To ensure a template's categories are not applied to a page using the template, template categories should be placed on the template's documentation page, normally after the two pipes in a <includeonly>{{Sandbox other||...}}</includeonly> block. Because template documentation is rarely protected, template categorization can be modified by all editors even when the template code itself is protected from editing by most editors. When there is no documentation page, the categories for the template may be placed at the bottom of the template itself, within a <noinclude>...</noinclude> block. There should be no spaces or new lines between the last part of the template proper and the opening <noinclude> tag.
User pages are not articles and thus do not belong in content categories such as Category:Living people or Category:Biologists. Similarly, draft versions of articles should be kept out of content categories. Also, do not transclude full articles into your user pages, as this will result in the user page being included in all the article's categories.
You also may not leave userspace pages sitting in red-linked categories that do not exist. Although it may seem less serious than mixing userspace content with mainspace content in conventional categories, red-linked categories in userspace get picked up by the category cleanup reports just the same as red-linked categories in mainspace, and thus actively disrupt the process of cleaning up mainspace category errors. The sole exception is that you are allowed to file your user page in Category:Wikipedians with red-linked categories on their user page, a harmless joke category that consensus has deemed a standalone exception to the rule–however, you must use that exact category itself, and are not free to use differently worded or differently spelled variants of it, and your page cannot be left in any other red-linked categories besides that.
Note also that Wikipedia policy around ownership of content means that other editors are allowed to edit pages in your personal userspace. If your page is in categories that it cannot be in under USERNOCAT, other editors are free to remove it from those categories and do not need to ask you for permission to edit "your" content.
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 administrative 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 customized per category; and ordering of categories on the page is less controllable.
When templates are used to populate administrative categories, ensure that the code cannot generate nonsensical or non-existent categories, particularly when the category name depends on a parameter.
Do not create inter-category redirects by adding #REDIRECT [[Target page]] to a category page. Unlike when renaming articles, categories cannot be redirected using "hard" redirects. Articles added to a redirected category will not show up in the target category. And since redirected categories do not become "red links", editors will not be aware when they add an article to a redirected category.
Instead, a form of "soft redirect" may be used in limited circumstances. You can create a category redirect by adding {{Category redirect|target}} to the category page. Bots patrol these categories and move articles to the "target" of the redirect.
If you need to add a redirect category to a {{Category redirect}}, use the template's second parameter. For example: {{Category redirect| Years of the 19th century in Ceylon |{{R from category navigation}}{{R from template-generated category}}}}
Category redirects are costly. While placing a page in a category redirect is not a big problem, it is a problem. There are plenty of helpful uses of category redirects, but category redirects are not as cheap as regular redirects. Links to article redirects are not broken: i.e. click on the link and you are automatically redirected to its target. However, links to category redirects are broken and need to be fixed, except when resolved by a template (i.e. the template generates categories using a pattern, and some of the resulting category names are intentionally resolved by the use of {{Resolve category redirect}} within the template, as in the Ceylon example above). Placing a page into a category redirect makes it show up in the category redirect–not its target–until a bot comes along and fixes the link.
Note: Usually only do this at an unused category name which is ambiguous, e.g. Category:Lewis. If an ambiguous name is used for an active category because it is clearly the primary topic, and there are only one or two other categories with similar names, use a hatnote like the one at Category:Lewes. For longer lists, we can also create pages such as Category:Bedford (disambiguation).
In cases where, for technical reasons, administrative 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–these are not hidden.
A logged-in user may elect to view all hidden categories, by checking "Show hidden categories" in the "Appearance" tab of their Preferences. "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" on the right-hand side, or by editing the whole page with the source editor.
Stub templates categorize stub articles for maintenance purposes into administrative categories, not for user browsing, so 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.
Partial view of Wikipedia's category system from 2007. Arrows point from category to subcategory.
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).
There is one top-level category: Category:Contents. All other categories are subcategorized below this category.
A tree structure showing the possible hierarchical organization of an encyclopedia
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 category (an is-a relationship), then the first category should be made a subcategory (directly or indirectly) of the second category (the "parent" category). Any category may potentially contain (or "branch into") subcategories.
A is said to be a parent category of B when B is a subcategory of A.
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.
If two categories are closely related but are not in a subset relation, then links between them can be included in the category description of the category pages.
Pages (and subcategories) should not usually be placed in both a given category and any of its subcategories or parent categories. For some exceptions to this, see §Eponymous categories and §Non-diffusing subcategories.
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.
Category chains formed by parent–child relationships should never form closed loops; that is, no category should be contained as a subcategory of one of its own subcategories.
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.
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. However, avoid creating subcategories for just one article.)
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 categories should be identified with a template on the category page:
Subcategories defined by gender, ethnicity, religion, and sexuality should almost always be non-diffusing subcategories to prevent othering. The Wikipedia:Categorizing articles about people guideline outlines the rules on these categories in more detail.
By default, a category page will list its member pages and subcategories alphabetically by their page titles. Sometimes, this is undesirable, so sort keys are needed to produce a better ordering. A sort key can be added to a page using {{DEFAULTSORT:sort key}}, which is a magic word that will cause the page to be sorted in categories under a different specified title. (Per MOS:ORDER, this is placed just before the list of category declarations.) The page title, and DEFAULTSORT, can also be overridden for individual categories using the same syntax as piped links: [[Category:Category name|sort key]]. For the mechanics, see Help:Category §Sorting category pages.
People are usually sorted by last name rather than first name, so "Last name, First name" sort keys are used (e.g. Washington, George for George Washington). There are many other rules for sorting people's names; for more information, see Wikipedia:Categorization/Sorting names.
Other sort key considerations (in no particular order):
In the English Wikipedia, sort order merges (ignores) letter case and diacritics. For example, "Baé", "Båf", and "BaG" would be sorted in that order. The same applies to some other variant letters and ligatures, e.g. Æ automatically sorts as AE, and ß automatically sorts as ss. However, others, such as Þ, do not have an automatic equivalent and may need to be replaced in sort keys with standard English letters to ensure logical ordering.
The main article(s) of a category, if existent, should have a space as the sort key so that they appear at the very top of the category (i.e. [[Category:Example| ]]). Those articles are typically identical or at least synonymous in name to their category. This is generally limited to one or two articles in a category.
Other general articles that are highly relevant to the category should be sorted with an asterisk as the key (i.e. [[Category:Example|*]]) so that they also appear near the top of a category but beneath the main articles. Those articles are typically called things like "List of example", "Outline of example", "Index of example", or similar. This is generally limited to a few articles in a category, and there may not be any. This is also often used to sort lists categories within relevant parent categories.
Use sort keys beginning with other symbols (e.g. -, +, etc.) for any other pages that should appear after the main article and before the ordinary alphabetical listings. The same technique is used to bring particular subcategories (such as metacategories) to the start of the list.
Sort order of characters before numbers and Latin alphabet (0–9, A–Z) is (partial list):
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 (e.g. {{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. words 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 as {{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 is not 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 as "Isle of Wight". Similarly, for settlements such as Isle of Wight, Virginia, the prefix is not 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 as {{DEFAULTSORT:Oneill, Eugene}}. All other punctuation marks and symbols should be removed. Dashes can be replaced with hyphens. (Commas can be added when re-ordering words, as in the previous example, and symbols can be added to the start to force the sort earlier, as explained above.)
A symbol used stylistically in the place of a letter should be replaced in a sort key with its standard letter equivalent. For example, Tight A$ has the sort key Tight As.
Numbers will sort numerically (e.g. 9 sorts before 12), but some entries containing numbers need special sort keys to ensure proper numerical ordering. It is important to stick to the same system for all similar entries in a given category.
Separators (such as commas or periods) in numbers will interfere with numerical sorting so should be removed in sort keys. For example, 10,000 Maniacs has the sort key 10000 Maniacs.
Roman numerals should be written as modern numbers in sort keys so that they sort numerically instead of alphabetically. For example, IX (9) comes before V (5) in alphabetical order, so Pope John V and Pope John IX should use the sort keys John 5 and John 9, respectively.
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"). 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.
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.
"β" (beta, displays as capital, "Β") is for barnstars.
"Δ" (capital delta) is for documentation, where sorting by Latin D is undesirable.
Similar to the handling of Latin letters, if the sort key begins with a lowercase Greek letter, then the capital Greek letter will be displayed in headings on category pages. 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.
Sort keys are sometimes used 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.
Anyone who can edit a page may 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 start or participate in a discussion about the category.
{{Improve categories}} 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}} when there are too many categories. Put this template at 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.
↑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 acyclic.
↑This includes punctuation-like letters with no English equivalent, such as ʻ.
↑Previously, zero padding was required to ensure single-digit numbers sorted before double-digit numbers (e.g. "09" for 9). This is no longer necessary.
↑"µ" (mu) was previously used, but the capital version "Μ" was confusing.
A tree structure showing the possible hierarchical organization of an encyclopedia
An item may belong to several subcategories of a parent category (as pictured).
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