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Categorisation is a feature of Wikia's software, enabling pages to be placed in categories which can then be used by readers to find sets of articles on related topics. Categories can be defined as subcategories of other categories, allowing easy navigation between connected subject areas via a tree-like structure. This helps readers find articles on particular topics even if they don't know which articles exist or what they are called.

How categories work[edit source]

Including in the text of any editable page causes that page to be placed in a category called Name. This link will not show up where it appears in the text, but a corresponding link will appear in the "Categories" box at the bottom of the page. The target of this link is a page called "Category:Name" – the link will be red until such a page is created. Conventionally, these category declarations are placed at the end of the page text, but before any stub templates (which themselves generate categories) and interlanguage links.

The category page (a page in the Category: namespace) is itself a normal editable page, but has a special mode of display – it lists all the pages that have been placed in the category it represents. If any of these pages is itself a category page, then it is treated as a subcategory; these are listed first. Otherwise it is treated as a member page of the category; these are listed below the subcategories. Any ordinary page content appearing in the text of the category page is displayed before the lists of subcategories and member pages. For further details about category pages, including sort order, see Display of category pages below.

To make a normal link to a category page from another page, precede the word "Category" with a colon, as in [[:Category:Music]], which appears as Category:Music. These links do not put the page in the category, and can be piped like ordinary wikilinks.

The category system [edit source]

Partial view of Wikipedia's category system. Definitely not a tree structure, but notice all the arrows point downwards.

Wikipedia's categories form a hierarchical structure, consisting in effect of overlapping trees. (Because subcategories can have more than one immediate parent, the system as a whole is not a tree, but rather approximates a directed acyclic graph.)

Categories are of two basic types:

  • topic categories – these contain articles on a particular topic; for example, Category:Tractors contains articles on subjects related to music.
  • list categories – these contain articles whose subjects are members of a particular set; for example, Category:companies contains articles on companies.

It is also possible to combine the two types, to create list-and-topic categories. For example, Category:Companies of Poland contains articles whose subjects are companies from poland, as well as articles relating to voivodeships in general.

If the articles of one category logically also belong to another category, then the first category is made a subcategory of the second. So for example, Category:Musicians might be made a subcategory of Category:Music (by adding [[Category:Music]] to the text of the page Category:Musicians – not the other way round). Notice that it is unlikely for a topic category to be a subcategory of a list category.

If B is a subcategory of A, then A is said to be a parent category of B.

Many topic categories are of the type called eponymous categories, i.e. they correspond to a main topic article of the same name. For details on how to treat these, see Eponymous categories below.

A page need not be added explicitly to all of the categories that logically contain it: normally a page (or subcategory) would not be added explicitly to a category if it is contained in one of that category's subcategories. However there are quite common exceptions to this: see Duplicate categorisation rule below.

Categorising pages[edit source]

Every Wiki article should belong to at least one category. Similarly every category (except Category:Contents, which is the root of the hierarchy) should be placed in at least one parent category. Disambiguation pages belong to special categories (see Disambiguation); most redirects are not categorised, though there are exceptions (see Categorising redirects). For the categorisation of pages in other namespaces, and categories used for project management purposes, see Project categories below.

An article should be placed in all the categories to which it logically belongs, subject to the duplicate categorisation rule stated below. It should be clear from the 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.

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 the section What categories should be created below. If you don't know where to put an article, add the {{uncategorised}} template to it – other editors (such as those monitoring Project:WikiProject Categories/uncategorised) will find good categories for it.

Categorise 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. (For exceptions, see Project categories below.)

An article should never be left with a non-existent (redlinked) category on it. Either the category should be created (most easily by clicking on the red link), or else the link should be removed or changed to a category that does exist.(rule not currently 'active' as we are restructuring category structure)As of March 2011

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.

What categories should be created[edit source]

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 Tractor & Construction Plant Wiki:Overcategorisation. Discussion about whether particular categories should exist takes place at Tractor & Construction Plant Wiki:Categories for discussion.

It should be remembered 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 the uses of these techniques, see Categories, lists and navigation templates.

Categories appear without annotations, so be aware of the need for a neutral point of view when creating or filling categories. If the composition of a category is likely to be controversial, a list (which can be annotated) may be more appropriate.

Before creating a new category, check whether a similar category does not already exist under a different name (for example, by looking on the likely member pages or in likely parent categories).

Categories follow the same general naming conventions as articles; for example, common nouns are not capitalised. For specific rules, see Tractor & Construction Plant Wiki:Naming conventions (categories).

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

Subcategorisation[edit source]


Although there is no limit on the size of categories, a large category will often be broken down 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 broken down using several coexisting schemes; for example, Category:Albums is broken down by artist, by date, by genre etc. Intermediate categories may be created as ways of organising schemes of subcategories. For example, the subcategories called "Artistname albums" are not placed directly into Category:Albums, but in the intermediate category Category:Albums by artist.

Not all subcategories serve this systematic "breaking down" function; some are simply subsets which have some 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 distinguished subcategories.

The identification of distinguished and non-distinguished subcategories is important for the application of the duplicate categorisation rule. It is useful to state in category descriptions whether or not a given category is a distinguished subcategory of a parent category. Use the {{allincluded}} and {{distinguished subcategory}} templates to specify the particulars. If no such information is present, determine the status of a subcategory by common sense and observation of the way existing articles are categorised.

Categories which are intended to be fully broken down into subcategories can be marked with the {{catdiffuse}} template. This 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 broken down into subcategories, you can add the {{verylarge}} template to the category page.

Subcategories defined by ethnicity and sexuality are often classed as distinguished. For example, Category:African American baseball players is a distinguished subcategory of Category:American baseball players, as this category is not broken down systematically by ethnicity. See also Wikipedia:Wikipedia:Categorization/Gender, race and sexuality.

Remember that subcategories will often belong in at least two 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. If two categories are closely related but are not in a subset relation, then a link to one can be included in the other's category description (see below).

Duplicate categorisation rule[edit source]


If a page is contained logically in both a category and a subcategory of that category, it must be considered whether it should still be placed directly into the first (parent) category. The rule normally applied is as follows, based on the definition of distinguished category as given above:

  • If category B is a non-distinguished subcategory of category A, then pages belonging to category B (directly or through further subcategories) are not placed directly into category A.
  • If category B is a distinguished subcategory of category A, then pages belonging to category B are placed directly into category A if otherwise appropriate.

For example, Angers Bridge is not placed directly into Category:Bridges, because it belongs to the non-distinguished (systematic) subcategory Category:Bridges in France. On the other hand, pages are not excluded from Category:Bridges in New York City on the grounds that they also belong to Category:Toll bridges in New York City, since the latter is a distinguished subcategory.

Eponymous categories[edit source]

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 in Ford Motor Company and Category:Ford. 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 order).

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. In any case, an article should not be excluded from any list category on the grounds that its eponymous category is made a "subcategory" of that category. For the purposes of the duplicate categorisation rule stated above, such eponymous categories are considered distinguished subcategories (in fact they are not true subcategories at all).

In other cases, eponymous categories have been categorised separately from their articles. In this case it will be helpful to readers if there are links between the category page containing the articles and the category page containing the eponymous categories. An example of this setup is the linked categories Category:American politicians and Category:Categories named after American politicians, using the template {{CatRel}}.

A clear link to the main topic article from an eponymous category page can be created using the template {{catmore}}.

Display of category pages[edit source]

Form of entries[edit source]

The entries as displayed on category pages are the exact names of the pages that have been placed in the category (minus the Category: prefix in the case of subcategories). It is not possible to change the way the entries are displayed using piping or any similar technique.

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 rather than 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 instead of on "John Smith". (Redirects appear in italics in category listings.) See also Tractor & Construction Plant Wiki:Categorising redirects.

Sort order[edit source]


It is possible to change the order of entries listed on a category page. This order is based on the sort key associated with each entry (notice that sort keys themselves are not displayed). If no sort key is defined explicitly for an entry, the sort key used is the page name as displayed.

Entries are intended to be arranged alphabetically, and the lists are broken down by initial character. However the "alphabet" used here is based on the Unicode character listing, and may give unexpected results. For example, all capital letters come before all lower case letters; modified letters come after all unmodified letters; and spaces come before anything else.

Using sort keys[edit source]

To change the position of an entry in the list, define an explicit sort key for that entry. This is done in the original category declaration on that entry's page, by adding the desired sort key after a pipe. For example, [[Category:Music|Trombone]] places the current page in the "Music" category with the sort key "Trombone".

If a page is to be given the same sort key in all or several of its categories, the {{DEFAULTSORT}} magic word can be used. The effect of the magic word does not depend on its position in the text, but it is conventional to place it just before the list of category declarations. For instance, on George Washington, type {{DEFAULTSORT:Washington, George}} to define "Washington, George" as the page's sort key for all categories, except any for which a different sort key is defined explicitly.

Default sort keys are often 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. Where a default sort key needs to be overridden with the name of the page, {{PAGENAME}} can be used as the sort key (this means that it will still work if the page is moved).

Sort keys are case sensitive, so care must be taken in specifying capitalisation. For example, do not begin a sort key with a lower case letter unless you want the article to appear on the category page separate from articles sorted with an upper case letter, under a lower case letter heading. A case-insensitive sort can be achieved by following the convention that initial letters of words are capitalized in the sort key, but other letters are lower case. For example, use "Dubois" in sort keys rather than "DuBois".

Typical sort keys[edit source]

  • Categories of people are usually sorted by last name rather than first name, so "surname, forename" sort keys are used as in the George Washington example above. For more information, see project:Categorisation of people#Ordering names in a category.
  • Entries containing modified letters should be sorted as if the letters were unmodified (for example, "Lodz" should be used as the sort key for Łódź).
  • Entries containing numbers sometimes need special sort keys to ensure numerical rather than alphabetical ordering (for example, 19 and 103 come before 2 in alphabetical order, and IX comes before V). So Haydn's 13th symphony might have the sort key "Symphony 013", the zero ensuring that it is listed before symphonies 100–108; Pope John IX might have a sort key "John 09". It is important to stick to the same system for all similar entries in a given category.
  • Similar systematic sort keys are used in other categories where the logical sort order is not alphabetical (for example, individual month articles in year categories such as Category:2004 use sort keys like "*2004-04" for April). Again, such systems must be used consistently within a category.
  • In some categories, sort keys are used to exclude prefixes that are common to all or many of the entries, or are considered unimportant (such as "List of" or "The"). For example, in Category:2004 the page 2004 in film would have the sort key "Film", and in Category:2004 in Canada the page 2004 Canadian federal budget would have the sort key "Federal Budget".
  • Use a space (" ") as the sort key for an article matching an Eponymous category, or a key article for the category. This ensures these appear at the start of the listing for that category, as in [[Category:Barack Obama| ]].
  • Use an asterisk ("*") for any "List of ..." and other pages that should appear after the key article and before the main alphabetical listings.
  • Similarly, in certain exceptional circumstances, subcategories may be grouped together at the beginning of a category listing by means of these special character sort keys. For example, in Category:Australia stubs general topics are grouped together by using a leading space as a sort key, and regional stubs are grouped together using a leading asterisk.
  • To place entries after the main alphabetical list, use sort keys beginning with tilde ("~"). Other characters used for this purpose are "µ", commonly used to place stub categories at the end of subcategory lists; "β" for book subcategories; and "τ" for categorising templates.

Split display[edit source]

When there are more than 200 entries in a category, only 200 are displayed on the screen at a time. Users can navigate between screens using the "previous 200" and "next 200" links provided. The text of the category page itself appears at the top of every screen. The URL for a category subpage with up to 200 entries listed alphabetically from a given point takes the following form:

(this example produces a page listing all the entries in Category:Living people alphabetically starting from "Aq").

To make navigating large categories easier, add a table of contents to the category page. This can be done using the following templates:

  • {{CategoryTOC}} – adds a complete table of contents (Top, 0–9, A–Z)
  • {{CatAZ}} – the same as {{CategoryTOC}}, but without the numbers 0–9
  • {{LargeCategoryTOC2}} – adds a complete table of contents with five subdivisions for each letter (Aa Ae Aj Ao At)
  • {{LargeCategoryTOC}} - adds a complete table of contents with twenty-six subdivisions for each letter (Aa ... Az)

Subcategories are split alphabetically along with the articles, which means that the initial screen of a split category may not include all its subcategories. To make all subcategories display on each screen, add a category tree to the text of the category page, as described below under Displaying category contents on pages.

Category description[edit source]

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 contains, how they should be subcategorised, and so on. The description can also contain links to other pages, in particular to other related categories which do not appear directly as subcategories or parent categories, and to "sister categories" on other projects, such as Commons.

Various templates have been developed to make it easier to produce category descriptions, such as {{catmore}}, {{cat see also}} and {{cat see also commons}}. For more of these, see Category namespace templates. Another technique that can be used is described at Tractor & Construction Plant Wiki:Classification.

Project categories[edit source]

The categories that readers are intended to see on article pages and use for browsing are called content categories. These are part of the encyclopedia and should be maintained as such, generally being kept separate from non-article pages and other categories which are addressed to editors rather than readers. Content categories are based on features of the subject matter of articles, not on an article's current state or other classifications specific to the Wiki project.

Categories which are not intended to serve as content categories are called project categories. There are various types of these, including 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 other namespaces. These categories are used by Wiki editors or automated tools, and do not aid readers' browsing.

Article pages should be kept out of project 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 a project 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 under Hiding categories below.

User pages[edit source]


User pages are not articles, and thus do not belong in content categories such as Category:Living people or Category:Biologists. They can however be placed in user categories – subcategories of Category:Wikians, such as Category:Wikian collectors – which assist collaboration between users. See Tractor & Construction Plant Wiki:User categories for further information.

Similarly, user subpages that are draft versions of articles should be kept out of content categories. 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.

Images[edit source]

Images are typically put in categories that contain only images. See Categorising images for more information. To find image categories, navigate from Category:Tractor Wiki images by subject or its parent category Category:Tractor Wiki images.

Hiding categories[edit source]

In cases where, for technical reasons, project 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 "uncategorised article" categories – these types are not hidden.

To hide a category, add the template {{hiddencat}} 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 "Misc" tab of My Preferences. Notice that "hidden" parent categories are never in fact hidden on category pages (although they are listed separately).

Categorisation using templates[edit source]

Many templates include category declarations in their transcludable text, for the purpose of placing the pages containing those templates into specific categories.

It is not normally recommended that articles be placed in ordinary content categories in this way. The technique is likely to lead to pages being miscategorised, or their categories being listed in an undesired order. In some instances, it may even have the unintended result of creating inappropriate categories, such as Category:Cities and towns in. Further, if such a template is used on the category page for the transcluded category, a confusing loop is generated, with the category listed as a subcategory of itself. However the technique is very commonly used for populating certain kinds of project categories, including stub categories.

Notice that changing the category in a template does not cause all pages with the template to be recategorised immediately – pages may need to be edited before their categorisation changes. This means that category lists for categories populated in this way may not always be up to date.

Category declarations in templates often use {{PAGENAME}} as the sort key, particularly if they are designed to be placed on talk pages, as this suppresses the prefix Talk: from the sort keys.

Redirected categories[edit source]

Although it is possible to attempt to redirect categories by adding a line such as #REDIRECT [[:Category:Automotive technologies]] to a category, it is not generally recommended because of limitations in the mediawiki software. Until these issues are addressed (in future versions of the software), #REDIRECT should not be added to category pages.

"Soft" redirects for categories can be created using {{Category redirect}}. A bot policy traverses categories redirected in this manner moving articles out of the redirected category into the target category; see Template talk:Category redirect.

Interlanguage links to categories[edit source]

Interlanguage links work just as they do for regular articles: [[de:Kategorie:Mathematik]] in Category:Mathematics connects to the German counterpart. This can be a useful way to compare coverage, or to look for articles in need of interlanguage links. Note that different Wikipedias may have adopted different standards and practices for categorisation, so not all categories have equivalents in other languages.

Tips[edit source]

Displaying category contents on pages[edit source]

To display the subcategory tree and (optionally) member pages of a given category on any page, use the CategoryTree extension (see the documentation page for full details). The basic syntax is

  • <categorytree>Category name</categorytree>

to display just the subcategory tree, and

  • <categorytree mode=pages>Category name</categorytree>

to display member pages as well.

Retrieving category information[edit source]

Raw information about the members of a category, their sortkeys and timestamps (time when last added to the category) can be obtained from the API, using a query of the form:


Linking to categories[edit source]

Outside mainspace, the following templates can be used to display a category in different ways, or link to its maintenance pages.

  1. {{cl|1984}} produces Cat:1984
  2. {{ccl|1984}} produces Category:1984
  3. {{lcs|1984}} produces Category:1984 (edit talk links history)

Searching for articles in categories[edit source]

In addition to browsing through hierarchies of categories, it is possible to use the search tool to find specific articles in specific categories. To search for articles in a specific category, type incategory:"CategoryName" in the search box. This can be used to find articles that are members of more than one category (see Tractor & Construction Plant Wiki:Category intersection for a proposal for a more sophisticated version of this feature). For example, enter the search text

incategory:"Suspension bridges" incategory:"Bridges in New York City"

to find the articles that are common to both categories — the suspension bridges in New York City — as here.

Similarly, an "OR" can be added to join the contents of one category with the contents of another. For example, enter

incategory:"Suspension bridges" OR incategory:"Bridges in New York City"

to return all pages that belong to either (or both) of the categories, as here.

Note that using search to find categories will not find articles which have been categorised using templates.

See also[edit source]

For browsing
For maintenance

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