User:Coder Dan/Categories
The category system
Wikipedia's categorization system is a hierarchical structure that contains sets of related articles called categories. It enables readers to search for articles on related topics. Editors specify the relationships between categories and articles on the corresponding pages, and Wikipedia's software automatically constructs the hierarchy. New categories can be added to the hierarchy, and classes of categories can be formed.
Wikipedia's category system forms a hierarchy of multiple, interconnected tree structures. The root category is called Contents. The trees are interconnected because each subcategory can have more than one parent. In technical terms, the hierarchy is a directed acyclic graph with a single source (Contents). It is commonly visualized as an upside-down tree, rooted at the top and branching downward, whose branches are allowed to merge.
The category hierarchy is typically visualized as a tree-like structure rooted at the top and branching downward into subcategories. Categories above any particular category are called the category's "ancestors", and the most immediate ancestor is called the "parent". The word category can refer to either the category page in the category hierarchy or the set of articles represented by that page.
Categorization is concerned with how articles are assigned to categories, and Subcategorization is concerned with how categories are related to each other. Both articles and category pages list their parent categories in a box at the bottom of the page. A category's subcategories are listed in the top section of the category page. For more concrete descriptions and techniques, see Category:Wikipedia help.
A category's title can describe the articles in the category by denoting either a set or a topic. If the title denotes a set, then articles in that category discuss members of that set. If the title denotes a topic, then the category's member articles are about subjects that pertain to that topic. For example, Music is a topic, and Musicians is a simple category. Music contains all articles related to music.
A category page lists its subcategories in the Subcategories section, and pages in the Pages or Media section. Articles and category pages both list parent categories in a box at the bottom of the page called the categories box. The content categories are divided conceptually into project, stub, and maintenance (category page) categories.
Subcategory is a (descendant) child category. The Subcategories section contains links to category pages.
Categorizing pages
Every Wikipedia article should belong to at least one category.
Articles are often listed in more than one category, but they are listed in only the lowest-level (most specific) categories possible. Higher-level category pages typically list only subcategories.
Each article should be placed in all of the childless (most specific) existing categories to which it logically belongs. 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.
- Disambiguation pages belong to special categories (see Disambiguation); most redirects are not categorized, though there are exceptions (see Categorizing redirects). For the categorization of pages in other namespaces, and categories used for project management purposes, see Project categories below.
- Normally a new article will fit into one or more 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 {{uncategorized}} template to it – other editors (such as those monitoring Wikipedia:WikiProject Categories/uncategorized) will find good categories for it.
- Categorize 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.
- Articles on fictional subjects should never be categorized in a manner that confuses them with real subjects. A set category such as Category:European countries or Category:Presidents of the United States should contain only real examples of those sets. If a set category for fictional subjects has a real-life counterpart, as with Category:Fictional Presidents of the United States, its contents should be expressly identified as fictional in the name of the category itself. This is not necessary where the grouping is purely fictional, as with Category:Klingons. Fictional subjects may be mixed with real ones only in topical categories. In topical categories, there is no risk of confusing fiction with fact as with list categories.
The order in which categories are listed in an article 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 important categories appear first. If an article has an eponymous category (see below), then that category should be listed first. For example, Category:George Orwell is listed before other categories in the George Orwell article.
Eponymous categories
The name of a category may be the same as the name of an article. This phenomenon is called eponomy.
Often an article and a topic category will have the same name, as in George W. Bush and Category:George W. Bush, or occasionally similar names referring to the same thing, as with Mekong and Category:Mekong River. Such a category is called eponymous. 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).
By convention, eponymous categories are an exception to the rule that topic categories should not be subcategories of set categories. Many eponymous categories are added along with their corresponding articles to the categories to which the article belongs. For example, Category:France is a subcategory of Category:European countries, which contains France, even though subjects pertaining to France are not themselves European countries.
In other cases, eponymous categories have been categorized 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}}.
What categories should be created
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 Wikipedia:Overcategorization. Discussion about whether particular categories should exist takes place at Wikipedia:Categories for discussion.
In addition to the category system, users may also use lists and navigation boxes to browse sets of related articles. For a comparison of the uses of these techniques, see Categories, lists and navigation templates.
Categorizations appear on pages without annotations or referencing to justify or explain their addition; editors should be conscious of the need to maintain a neutral point of view when creating categories or adding them to articles. Categorizations should generally be uncontroversial; if the category's topic is likely to spark controversy then a list article (which can be annotated and referenced) is likely to be more appropriate.
Before creating a new category, check whether a similar category already exists 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 capitalized. For specific rules, see Wikipedia:Naming conventions (categories).
For proposals to delete or rename categories, follow the instructions at Categories for discussion.
Subcategorization
In addition to grouping related articles into categories, the category system groups related categories into larger, more general categories. A category that contains other categories is called the "parent" of those categories, and the categories contained by the parent are called its "children" or "subcategories".
There is one category, called "Contents", that contains all Wikipedia content and all other categories. Specific categories can be found by following the appropriate child links from Contents, and general categories can be found from more specific categories by following parent links. The parent-child relationships between categories create a web of interconnected chains of categories that indirectly relate more general categories to more specific ones. Each category can have more than one parent, so the category hierarchy created by these chains is not a simple tree structure.
If a group of subcategories collectively contain every member article of their parent exactly once, they can be called a "full", "complete", "exhaustive", "comprehensive", or "systematic" set of subcategories. Such a set of subcategories systematically subdivides the parent category. They may be placed in their own Wikipedia page, which is listed in the parent category's page in place of the individual subcategories. If this is done, the separated set of subcategories can be called a "category list", "subcategory set", or "metacategory".
Any parent category that does not contain one or more systematic sets of subcategories should list at least any member articles that are not contained in at least one subcategory. Subcategories that are not part of a systematic set are called "ad hoc" or "standalone" subcategories.
Subcategorization issues include duplication, circularly referencing branches, and over-categorization (a sort of notability for categories). As articles are added to a category, new subcategories may be created to hold the articles. This prevents individual category pages from becoming too big. Each category page has a lead section with a layout similar to that of an article, except the lead section of the category page may contain classifications. Every new category should be placed in at least one parent category.
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.
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.
A topic category may contain anything related to that topic, but a set category contains only members of that set. Therefore, topic categories may contain set categories, but set categories should not contain topic categories.
Subcategories will often belong in more than one parent category. 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 subcategory really can be expected (with possibly a few exceptions) to belong to the parent. 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).
The category Humans belongs to the category Primates. For the Human category to appear in the Primate category, the Category:Humans page makes the category declaration [[Category:Primates]]
If B is a subcategory of A, then A is said to be a parent category of B. The branch Humans-Primates-Mammals-Vertebrates contains four categories. If the category declaration in a category page is an arrow from itself (the subcategorized category) to the "parent" category, and if the root category is at the bottom, all the arrows point downward. (See the figure and its caption.) The direction of a category branch (a sequence of logical categorizations of pages) is counter-intuitive. Of the four category pages in the branch given, the first three contain the category declarations that make them subcategories, and the fourth category page does not contain a category declaration that points toward the other three. Vertebrates is a parent category of this branch.