This page documents an English Wikipedia editing guideline. It is a generally accepted standard that editors should attempt to follow, though it is best treated with common sense, and occasional exceptions may apply. Any substantive edit to this page should reflect consensus. When in doubt, discuss first on the talk page. Shortcuts:
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This page contains 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, try the categorization FAQ. For the naming of categories, see Wikipedia:Category names.

The category system

Wikipedia's category system can be thought of as consisting of overlapping trees. Any category may branch into subcategories, and it is possible for a category to be a subcategory of more than one parent. (A is said to be a parent category of B when B is a subcategory of A). Mathematically speaking, this means that the system approximates a directed acyclic graph.

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.

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, then the first category should be made a subcategory (directly or indirectly) of the second. For example, Cities in France is a subcategory of Settlements in France, which in turn is a subcategory of Geography of France.

Categorizing pages

Which pages a given category is supposed to contain is generally clear from the name of the category. Additional explanation may be placed in the text of the category page.

Pages are not placed directly into every possible category, only into the most specific one in any branch. 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.

Every Wikipedia article should belong to at least one category. Normally a new article will fit into existing categories – compare articles on similar topics to find what those categories are. If you think a new category needs to be created, see What categories should be created below. If you don't know where to put an article, add the {{uncategorized}} template to it – other editors (such as those monitoring Wikipedia:WikiProject Categories/uncategorized) will find good categories for it.

For information on whether and how to categorize pages other than articles, see Non-article and maintenance categories below.

Particular considerations for categorizing articles:

• It should be clear from verifiable information in the article why it was placed in each of its categories. Use the {{Category unsourced}} template if you find an article in a category that is not shown by sources to be appropriate, or the {{Category relevant?}} template if the article gives no clear indication for inclusion in a category.
• Categorize articles by characteristics of the topic, not characteristics of the article. A biographical article about a specific person, for example, does not belong in Category:Biography.
• Articles on fictional subjects should not be categorized in a manner that confuses them with real subjects.
• 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.
• By convention, category declarations are placed at the end of the wikitext, but before any stub templates (which themselves transclude categories) and interlanguage links.
• 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. If an article has an eponymous category (see below), then that category should be listed first of all. For example, Category:George Orwell is listed before other categories on the George Orwell page.
• If the desired display text in a particular category is different from the title of the article, it may be appropriate to categorize a redirect as well as the article itself. For example, if John Smith was notable as both a musician and a writer, but used the stage name Johnny Rocket in his musical career, then the musician category declarations can be placed on the "Johnny Rocket" redirect page as well as on "John Smith". (Redirects appear in italics in category listings.) See also Wikipedia:Categorizing redirects.

What categories should be created

Categories should be useful for readers to find and navigate sets of related articles. They should be the categories under which readers would most likely look if they were not sure of where to find an article on a given subject. They should be based on essential, "defining" features of article subjects, such as nationality or notable profession (in the case of people), type of location or region (in the case of places), etc. Do not create categories based on incidental or subjective features. Examples of types of categories which should not be created can be found at Wikipedia:Overcategorization.

Remember that categories are not the only means of enabling users to browse sets of related articles. 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 Categories, lists and navigation templates.

Categorizations appear on 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.

Before creating a new category, check whether a similar category already exists under a different name (for example, by looking on the likely member pages or in likely parent categories).

Categories follow the same general naming rules as articles; for example, common nouns are not automatically capitalized. For specific rules, see Wikipedia:Naming conventions (categories).

For proposals to delete or rename categories, follow the instructions at Categories for discussion.

Subcategorization

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WP:SUBCAT

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. When making one category a subcategory of another, ensure that the members of the first really can be expected (with possibly a few exceptions) to belong to the second also. Category chains formed by parent-child relationships should never form closed loops. 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.

Diffusing large categories

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 Rivers of Albania, Rivers of Andorra, etc.

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.

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 {{catdiffuse}} 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 {{verylarge}} template to the category page.

Non-diffusing subcategories

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WP:DUPCAT

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, Toll bridges in New York City as a subcategory of Bridges in New York City, and Musical films as a subcategory of Musicals. These are called non-diffusing subcategories. They sometimes 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.)

It is useful to identify non-diffusing subcategories with a note on the category page. The {{allincluded}} and {{distinguished subcategory}} templates can be used.

Subcategories defined by ethnicity and sexuality are often non-diffusing subcategories. See also the gender, race and sexuality categorization guideline.

Eponymous categories

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WP:EPON

Often an article and a topic category will share the same name, as in George W. Bush and Category:George W. Bush, or occasionally similar names referring to the same thing, as with Mekong and Category:Mekong River. Such a category is called an eponymous category. Naturally the article itself will be a member of the category (and should be sorted to appear at the start of the listing, as described below under Sort keys). A clear link to the main topic article from an eponymous category page can be created using the template {{catmore}}.

The question arises as to whether eponymous categories should be placed in (made subcategories of) the categories which their corresponding articles belong to. Logically they usually should not (for example, France belongs to Category:European countries, but Category:France does not constitute a subset of European countries). However, by convention, many categories do contain their articles' eponymous categories as subcategories, even though they are not "true" subcategories. In any case, an article should not be excluded from any set category on the grounds that its eponymous category is made a "subcategory" of that category.