When to use categories
Every page in the article namespace should belong to at least one category. Categories should be on major topics that are likely to be useful to someone reading the article.
Questions to ask to know whether it is appropriate to add an article to a category:
- If the category does not already exist, is it possible to write a few paragraphs or more on the subject of the category, explaining it?
- If you go to the article from the category, will it be obvious why it's there? Is the category subject prominently discussed in the article?
If the answer to either of these questions is no, then the category is probably inappropriate. Note that it is always appropriate to add articles to categories that fit into well established taxonomies.
Each Wikipedia article can appear in more than one category, and each category can appear in more than one parent category. Multiple categorization schemes co-exist simultaneously. In other words, categories do not form a strict hierarchy or tree structure, but a more general directed acyclic graph (or close to it, see below).
Categories appear without annotations, so be careful of NPOV when creating or filling categories. Unless it is self-evident and uncontroversial that something belongs in a category, it should not be put into a category.
Categories vs. Lists vs. Info boxes
See, Wikipedia:Categories, lists, and series boxes.
How to create categories
Creating a category is as simple as adding a soft link to the appropriate article in the Category: namespace; for instance, to add Felis silvestris catus to the "fluffy creatures" category, you would edit the article and enter [[Category:Fluffy creatures]] at the bottom, but before interlanguage links. Although the link will not appear in the article text, a page called Category:Fluffy creatures will automatically list alphabetically all articles that contain the [[Category:Fluffy creatures]] link. The appeal of categories is that unlike lists, they update themselves automatically, and that one can use them to quickly find related articles. However, categories are not a substitute for lists, and you will find that many articles belong to both lists and categories. You may see some inconsistencies when first creating the category: it may alternate between appearing empty and appearing with your first additions. It will probably correct itself in a few minutes.
Note that, although "uncreated" categories will correctly list articles that have been assigned to them, the category page itself does not exist until it is manually created. The easiest way to create the category page is to follow the edit link from an article and add a parent category and a category description as explained below.
Creating subcategories
Create subcategory pages by putting the name of the parent category on a category page that you would like to be the subcategory. Child categories (subcategories) are created by putting [[category:parent_category_name]] on the lower-level category pages. For example, on a (sub)category page called category:Roses you put [[category:Flowers]], Roses becomes a subcategory of Flowers.
When adding an article to a category, or creating categories, one should be careful to use the correct categories and subcategories. Horizontal categorization, directly below, refers to placing an article in the correct category while vertical categorization refers to placing an article in the correct subcategory. When assigning an article into categories, try to be thorough in a "horizontal" sense. A good general rule is that articles should be placed in the most specific categories they reasonably fit in.
For instance, the instance Dracula does not belong to the category "Myths and Legends" but "Classic vampires" as it is only recently that the historical character has been associated with vampirism.
Whatever categories you add, make sure they do not implicitly violate the neutral point of view policy. If the nature of something is in dispute (like whether or not it's fictional or scientific or whatever), you may want to avoid labelling it or mark the categorization as disputed. Most categorizations are pretty straightforward, though.
Making groups of subcategories
Categories can only list 200 entries at a time. When there are more than 200 entries, only the first 200 will be displayed. To make it easy to navigate, add a TOC (table of contents). TOCs are added by typing:
- {{CategoryTOC}} - which adds a complete TOC (Top, 0 - 9, A-Z)
- {{CatAZ}} - which adds a TOC without numbers. This is for categories with members that only start with letters.
When a given category gets crowded, also consider making several subcategories. Group similar articles together in a meaningful and useful way that will make it easy for readers to navigate later. Remember that several subcategorization schemes can coexist (for example, if Category:Software gets too big, you don't have to choose between subdividing it by function or subdividing it by platform, you can simultaneously subdivide it in both ways).
Category naming
Categories follow the same general naming conventions as articles, for example do not capitalise regular nouns. For specific conventions related to categories, see Wikipedia:Naming conventions (categories).
Categories requirements and usage
User browsing
Categories (along with other features, like cross-references) should help users find the information they are looking for as quickly as possible, even if they don't know that it exists or what it's called.
Links to categories
You can create a link to a category page without adding the page to that category by using a colon before the word Category. Example: [[:Category:Spirits] appears as Category:Spirits.
Redirected categories
Although it is possible to attempt to redirect categories by adding a line such as #REDIRECT [[:Category:Spirits]] to a category, it is not generally recommended because of limitations in the mediawiki software. Categories "redirected" in this way do not prevent the addition of articles to the redirected category. Articles added to the "redirected" category do not show up as in the target category. Until these issues are addressed (in future versions of the software), #REDIRECT should not be added to category pages.
Category sorting
Contrary to some expectations, text after a pipe ("|") in a category link is not used in place of the category text. Rather, this text is used as the sort key on the category page itself. However, again contrary to expectations, that sort text is not displayed.
For example, the Category:Three-digit Interstate Highways uses this property to sort secondary interstates by their primary. That is, the category link in the article for Interstate 190 (Illinois) is [[Category:Three-digit Interstate Highways|90-1 Illinois]]. This causes "Interstate 190 (Illinois)" to be listed right after "Interstate 189" and right before "Interstate 290 (Massachusetts)" under the heading "9" in the category page.
This feature is very useful for categories in which:
- the entries are people, in which case sorting by last name is preferable (example: Category:Presidents of the United States).
- variations in the naming of entries causes inconsistent sorting (example: List of counties in Nebraska, List of counties in Montana, but List of Nevada counties in Category:Lists of U.S. counties).
- the logical sort order is not alphabetical (example: individual month articles in year categories such as Category:2004, using e.g. the sort key "*2004-04" for April).
Using this method to sort category entries is sometimes informally referred to as the pipe trick. However, this use of the pipe character is very different from the original Wikimedia pipe trick which allows one to easily hide parenthetical disambiguation in links.
It is possible to force an article or subcategory to the top of the list by using a non-alphanumeric character as the first after the pipe. For example, using [[Category:Ford| Ford Motor Company]] (note the space) or [[Category:Ford|*Ford Motor Company]] would force that article to be displayed before all the others.