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, see the Categorization FAQ. For proposals to delete, merge, or rename categories, follow the instructions at Categories for discussion. Please use it before undertaking any complicated re-categorization of existing categories or mass creation of new categories.
The central goal of the category system is to provide navigational links to Wikipedia pages in a hierarchy of categories which readers, knowing essential—defining—characteristics of a topic, can browse and quickly find sets of pages on topics that are defined by those characteristics.
Categories are not the only means of enabling users to browse sets of related pages. Other tools which may be used instead of or alongside categories in particular instances include lists and navigation boxes. For a comparison of these techniques, see Categories, lists and navigation templates.
After you have determined an appropriate category name and know its parent category, you are ready to create the new category. To create a category, first add an article to that category. Do this by editing the article page. At the bottom, but before the interwiki links (if any are present), add the name of the new category, (e.g.:
[[Category:New category name]] ), and save your edit. The as-yet-undefined category name will now appear as a red link in the article's category list at the bottom of the page.
Next, to create the category, click on that red link, which brings you back into the editor. Adding this new category into the appropriate parent category is much the same as with an article: at the bottom, simply add the parent category (e.g.:
[[Category:Parent category name]] ), which should usually be a hypernym of the sub-category.
Sometimes, a common-sense guess based on the title of the category isn't enough to figure out whether a page should be listed in the category. So, rather than leave the text of a category page empty (containing only parent category declarations), it is helpful – to both readers and editors – to include a description of the category, indicating what pages it should contain, how they should be subcategorized, and so on.
In such cases, the desired contents of the category should be described on the category page, similar to how the 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.
A maximum of 200 category entries are displayed per screen. To make navigating large categories easier, a table of contents can be used on the category page. The following templates are some of the ways of doing this:
Subcategories are split alphabetically along with the articles, which means that the initial screen of a split category may not include all its subcategories. To make all subcategories display on each screen, add a category tree to the text of the category page, as described at the help page under 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. (However, there is no need to categorize talk pages, redirects, or user pages, though these may be placed in categories where appropriate.) In addition, each categorized page should be placed in all of the most specific categories to which it logically belongs. This means that if a page belongs to a 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 hierarchy, do not add categories to pages as if they are tags.
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.
A central concept used in categorizing articles is that of the defining characteristics of a subject of the article. A defining characteristic is one that reliable sources commonly and consistently define the subject as having—such as nationality or notable profession (in the case of people), type of location or region (in the case of places), etc. For example, in Caravaggio, an Italian artist of the Baroque movement, Italian, artist, and Baroque may all be considered to be defining characteristics of the subject Caravaggio.
An eponymous category should have only the categories of its article that are relevant to the category's content. For example:
Category tags can be added to file/image pages of files that have been uploaded to Wikipedia. When categorized, files are not included in the count of articles in the category, but are displayed in a separate section with a thumbnail and the name for each. A category can mix articles and images, or a separate file/image category can be created. A file category is typically a subcategory of the general category about the same subject, and a subcategory of the wider category for files, Category:Wikipedia files. To categorize a new file when uploading, simply add the category tag to the upload summary.
Freely licensed files may also be uploaded to, and categorized on, Wikimedia Commons. This can be done instead of, or in addition to, uploading and categorizing on Wikipedia. Most freely licensed files will eventually be copied or moved from Wikipedia to Commons, with a mirror page remaining on Wikipedia. (For an example of one such mirror page, see 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.
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.
In maintenance categories and other administrative categories, pages may be included regardless of type. To sort each namespace separately, see § Sort keys. E.g. 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.
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, 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.
At Database reports/Polluted categories, a list of affected categories is maintained.
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 templates, 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).
Many templates include category declarations in their transcludable text, for the purpose of . This technique is very commonly used for populating certain kinds of administration categories, including stub categories and maintenance categories.
However, it is recommended that articles not be placed in ordinary content categories using templates in this way. There are many reasons for this: editors cannot see the category in the wikitext; removing or restructuring the category is made more difficult (partly because automated processes will not work); inappropriate articles and non-article pages may get added to the category; sort keys may be unavailable to be customised per category; and ordering of categories on the page is less controllable.
When templates are used to populate administration categories, ensure that the code cannot generate nonsensical or non-existent categories, particularly when the category name depends on a parameter. Also, see Category suppression for ways of keeping inappropriate pages out of template-generated categories.
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.
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 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 Sorting category pages on the help page.
Categories of people are usually sorted by last name rather than first name, so "surname, forename" sort keys are used (as in "Washington, George"). There are many other rules for sorting people's names; for more information, see WP:NAMESORT.
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.) 
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.
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; that is, no category should be contained as a subcategory of one of its own subcategories. If two categories are closely related but are not in a subset relation, then links between them can be included in the text of the category pages.
Apart from certain exceptions (i.e. non-diffusing subcategories, see below), an article should be categorised as low down in the category hierarchy as possible, without duplication in parent categories above it. In other words, a page or category should rarely be placed in both a category and a subcategory or parent category (supercategory) of that category (unless the child category is non-diffusing – see below – or eponymous). For example, the article "Paris" need only be placed in "Category:Cities in France", not in both "Category:Cities in France" and "Category:Populated places in France". Because the first category (cities) is in the second category (populated places), readers are already given the information that Paris is a populated place in France by it being a city in France.
Sometimes proper subcategorization requires the 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 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. (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.
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 or Musical films as a subcategory of Musicals. These are called non-diffusing subcategories. They provide an exception to the general rule that pages are not placed in both a category and its subcategory: there is no need to take pages out of the parent category purely because of their membership of a non-diffusing subcategory. (Of course, if the pages also belong to other subcategories that do cause diffusion, then they will not appear in the parent category directly.)
Non-diffusing subcategories should be identified with a template on the category page:
Subcategories defined by gender, ethnicity, religion, and sexuality should almost always be non-diffusing subcategories. The categorization guideline outlines the rules on these categories in more detail.
Note that some categories can be non-diffusing on some parents, and diffusing on others. For example, Category:British women novelists is a non-diffusing sub-category of Category:British novelists, but it is a diffusing subcategory of Category:Women novelists by nationality.