This page documents an English Wikipedia editing guideline. It is a generally accepted standard that editors should attempt to follow, though it is best treated with common sense and the occasional exception. Any substantive edit to this page should reflect consensus. When in doubt, discuss first on the talk page.

Shortcut: WP:SUBCAT

Subcategories are often created in order to break down the members of a category into smaller sets which are easier to browse. This means that articles belonging to such a subcategory should not be placed in the parent category also. For example, Category:Bridges is broken down by country, so Angers Bridge appears in Category:Bridges in France, but not in the parent Category:Bridges.

In other cases, however, it is appropriate to place articles in a parent category as well as a subcategory. This is known as "duplicate categorization". This normally occurs when the subcategory refers to a distinguished feature of certain members of the parent category, but is not part of a complete systematic breakdown of that category. For example, the members of Category:Best Actor Academy Award winners should also appear in its parent Category:Film actors.

[edit] Reasons for duplication

There are some good reasons why duplication may be a benefit:

1. The basic principle is that duplication should make it easier, not harder, for users to find articles.
2. Different users may think of different categories when they look up their targets, multiple listing or locations enable users to spot their target easily
3. Wikipedia's categorization scheme allows for multiple taxonomies. This is a good thing and a powerful feature. The categories act as indexes, facilitating users to browse through subjects.
4. There might be articles that belong in a subcategory, while not belonging in a parent category; without duplication, users might assume that all articles in the subcategory belong in the parent category, when this may not actually be the case. (See the "user benefit" rule in the next section.)

[edit] When to duplicate

When deciding for or against duplication, one should bear in mind the potential benefits as described above. It is not essential that there be consistency across all of Wikipedia's categories, since each case or situation may differ. It is important that the duplication makes sense for the subject matter. Below are described some situations where duplication is appropriate.

[edit] Incomplete sets of subcategories

Duplication is usually used when the subcategory is not part of a complete categorization scheme for members of the parent category. For example, actor Marlon Brando is in both Category:Best Actor Academy Award winners and its parent Category:Film actors.

While it is possible to add Category:Actors who never won an Oscar to complete the categorization scheme with all actors being in one or the other of the subcategories and none in the parent category, this wouldn't make the categories any more useful, and would make it much harder to categorize actors and search for them in categories.

Another example of this is Category:Bridges in New York City and Category:Toll bridges in New York City. All the toll bridges are listed in both categories. There would be no advantage in completing the scheme by creating a category of non-toll bridges, nor in excluding the toll bridges from the category of all bridges.

Subcategories defined by ethnicity and sexuality are often of this type. For example, there is a category Category:African American baseball players, but the parent category Category:American baseball players is not broken down systematically by ethnicity. Therefore the members of the former category should also belong to the latter. See Wikipedia:Categorization/Gender, race and sexuality for more information.

[edit] Topic articles

When an article is the topic article for a category, it should be placed in that category. However, the article and the category do not have to be categorized the same way. The article belongs in categories populated with similar articles. The category should be put into categories populated with similar subcategories. For example, see George W. Bush and Category:George W. Bush.

If, however, the topic article and the similarly named category come to be placed in the same parent category, the fact that the article is a member of this subcategory is not a reason for it to be excluded from the parent category. Here, the double listing tells users that there is an article about the topic, and there are also more articles to be found in the subcategory of the same name. It makes it easier to find main topic articles (by eliminating the need to go to the subcategory). It also creates a complete listing of articles at the higher level category. It points readers of the topic article to the category and vice versa.

[edit] User benefit

If the removal of duplication would make it harder for readers to browse through subjects or spot their target easily, then duplication should not be removed.

For example, consider Category:Musicals and its subcategory Category:Musical films. Some musicals have separate articles for the stage and screen productions, while sometimes the same article covers both. Since the articles are not written in a consistent way, the duplication helps users find similar articles when browsing through either category.

[edit] Determining the existing status

If the status of a subcategory (as duplicated or unduplicated) is already established, then editors categorizing new articles should not normally change that status without good reason. The following questions will help decide if duplicate categorization is established:

• Are there any explanations posted in the categories that explain what goes in each category? Many categories will say right on the top of the page what belongs in the category. Sometimes there is a discussion on the talk page that can help make a decision. Ideally all categorization schemes that have duplications will have instructions to editors about how things should be categorized.
• Are other articles that are in subcategories also in the higher category? If there are no articles in the category, and its subcategories are well populated, then probably all the entries should be in subcategories only. An example of this is Category:Entertainers.
• If there are entries in the parent category, could they be there by mistake? Sometimes there are actors in Category:Entertainers, but the few that are there have been put there by mistake and most likely fairly recently. All the other actors have been put in subcategories and all that is left is a small random selection of entries. However if there is a broad selection of entries in both category and subcategory, then duplication is probably deliberate.