==Using MediaWiki Categories==

An overview of how categories are used, both on this wiki and in MediaWiki in general.

===What are categories used for?===

Categorization of articles serves a handful of purposes:

* Category trees make large numbers of articles more browsable by grouping them hierarchically.
* The parent category or categories of an article provides one means of finding related articles.
* The categories an article is in can provide information about the article itself.

For example, the [[px]] article is in [[:Category:Perforce]] and [[:Category:C/C++]].  Even without knowing anything else about it, these categorizations tell us that this project is a Perforce utility, and that it was written in C or C++.  The category links on that page give us an easy means to navigate from that page to other projects that may be similar in either application or implementation.

===How do categories work?===

Categories in MediaWiki are implemented as a series of links and dynamically generated pages, rather than as a single directory structure, since most wiki articles are created at the top level of the wiki namespace.

A page is added to a category with the same syntax that is usually used to create links to other wiki pages:

 <nowiki>[[Category:</nowiki>'''CategoryName''']]

Rather than generating a link in the body of the article, a category link generated in this way is placed at the bottom of the page.  (Note: a normal link to a category page can be created by placing a ''':''' before the word '''Category'''.)  A page can belong to any number of categories, and typically any page should belong to at least one.

When a page is included in a category, the page representing that category automatically generates a link back to the including page; this automatic reciprocal linking is what makes it possible to navigate a tree of category pages.

If a page is included in a category that does not yet exist, the category link will be in the usual "nonexistent" style (red in most MediaWiki skins).  Although the important part of a category page (the links to articles in that category) is generated dynamically, a category page can also be edited like any other wiki page.  Text added to a category page is displayed at the top before the dynamically generated listing.

Categories can (and should) themselves be included in other categories; this is done by adding the usual <nowiki>[[Category:</nowiki>'''CategoryName''']] to the category page.  Typically each category has at least one parent category, and all articles and categories have a chain of inclusion that goes back to a single top-level category ([[:Category:Contents]] on this wiki).

Like articles, categories can be members of multiple categories.  An example on this wiki is [[:Category:Perforce web APIs]], which represents an intersection of two different categories and is therefore a member of both of them.

{{Advanced Mediawiki usage|Categories}}