Using_Categories.md #1

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](px "wikilink") article is in
[:Category:Perforce](:Category:Perforce "wikilink") and
[:Category:C/C++](:Category:C/C++ "wikilink"). 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:

`[[Category:`**`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 \[\[Category:**CategoryName**\]\] 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](:Category:Contents "wikilink") on this wiki).

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