Editing Incubator escapee wiki:Categorization (section)

==Category tree organization==
{{Shortcut|WP:CAT#TREE}}{{see also|Category:Contents|Tree structure|Tree (data structure)}}
[[File:Category-diagram.png|thumb|300px|Partial view of Wikipedia's category system from 2007. Arrows point from category to subcategory.]]
Categories are organized as overlapping "[[Tree structure|trees]]", formed by creating links between inter-related categories (in mathematics or computer science this structure is called <!---Wikipedia's categories don't have unique joins or meets---a lattice or---> a [[partially ordered set]]). 

There is one top-level category: [[:Category:Contents]]. All other categories are subcategorized below this category.
{{Gallery
|title=
|File:Binary tree structure.svg
|A [[tree structure]] showing the possible hierarchical organization of an encyclopedia
|File:Bureaucracy(medium).svg
|
|File:Tree product pract.svg
|An item may belong to several subcategories of a parent category (as pictured).
|}}

{{anchor|SUBCAT|PARENTCAT}}
=== Subcategorization ===
{{shortcut|WP:SUBCAT|WP:PARENTCAT}}

If logical membership of one category implies logical membership of a second category (an [[is-a]] relationship), then the first category should be made a subcategory (directly or indirectly) of the second category (the "parent" category). Any category may potentially contain (or "branch into") subcategories. 
* For example, A is said to be a ''parent category'' of B when B is a ''subcategory'' of A.) <ref>Mathematically speaking, this means that the system approximates a [[directed acyclic graph]].</ref>

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 also.
*For example, [[:Category:Cities in France|Cities in France]] is a subcategory of [[:Category:Populated places in France|Populated places in France]], which in turn is a subcategory of [[:Category:Geography of France|Geography of France]].

A subcategory may potentially have two or more parent categories.
*For example, [[:Category:British writers]] would be in both [[:Category:Writers by nationality]] and [[:Category:British people by occupation]].

If two categories are closely related but are not in a subset relation, then links between them can be included in the [[#CATDESC|category description]] of the category pages.

Category [[Chain (ordered set)|chains]] formed by parent–child relationships should never form closed loops;<ref>This condition can be formulated in terms of [[graph theory]] as follows: the directed graph that has the categories as vertices and the parent-child relationships as edges should be ''[[directed acyclic graph|acyclic]]''.</ref> that is, no category should be contained as a subcategory of one of its own subcategories.{{#tag:ref|There is an exception to this for maintenance purposes. For example, [[:Category:Hidden categories]] is a direct subcategory of itself and of [[:Category:Wikipedia extended-confirmed-protected pages]] and [[:Category:Container categories]], each of which is a direct subcategory of [[:Category:Hidden categories]].}} 

{{anchor|DIFFUSE}}
===Diffusing large categories===
{{Shortcut|WP:DIFFUSE}}{{redirect|WP:DIFFUSE|conflicts|Wikipedia:Diffusing conflict}}
Although there is no limit on the size of categories, a large category will often be broken down ("diffused") into smaller, more specific subcategories. For example, [[:Category:Rivers of Europe]] contains no articles about specific rivers directly; they are all in subcategories.

{{anchor|Metacategories|META}}A category may be diffused using several coexisting schemes; for example, [[:Category:Albums]] is broken down by artist, by date, by genre etc. '''Metacategories''' may be created as ways of organizing schemes of subcategories. For example, the subcategories called "''Artistname'' albums" are not placed directly into [[:Category:Albums]], but into the metacategory [[:Category:Albums by artist]], which itself appears in Category:Albums. (See [[:Category:Categories by parameter]])

It is possible for a category to be only partially diffused—some members are placed in subcategories, while others remain in the main category.

Information about how a category is diffused may be given on the category page. Categories which are intended to be fully broken down into subcategories can be marked with the {{tl|category diffuse}} template, which indicates that any pages which editors might add to the main category should be moved to the appropriate subcategories when sufficient information is available. (If the proper subcategory for an article does not exist yet, either create the subcategory or leave the article in the parent category for the time being.)

To suggest that a category is so large that it ought to be diffused, or substantially diffused, into subcategories, you can add the {{tl|overpopulated category}} template to the category page.

{{anchor|NON-DIFFUSE|NONDIFFUSE|ALLINCLUDED|DUPCAT}}

===Non-diffusing subcategories===
{{Shortcut|WP:CAT#NON-DIFFUSE|WP:ALLINCLUDED|WP:DUPCAT}}
Not all subcategories serve the "diffusion" function described above; some are simply subsets which have some special characteristic of interest, such as [[:Category:Best Actor Academy Award winners|Best Actor Academy Award winners]] as a subcategory of [[:Category:Film actors|Film actors]]. They provide an exception to the general rule that pages are not placed in both a category and its subcategory: there is no need to take pages out of the parent category ''purely'' because of their membership of a non-diffusing subcategory. (Of course, if the pages also belong to other subcategories that do cause diffusion, then they will not appear in the parent category directly.)

Non-diffusing subcategories should be identified with a template on the category page:
* The {{Tl|Non-diffusing subcategory}} templates should be used for subcategories that are non-diffusing, like {{cl|American novelists of Asian descent}}.
* The {{Tl|All included}} can be used for categories where all child articles of a certain type are in the parent, like {{cl|Presidents of the United States}} or {{cl|Mountains of Switzerland}}.

Subcategories defined by gender, ethnicity, religion, and sexuality should almost always be non-diffusing subcategories to prevent [[Other (philosophy)|othering]]. The [[Wikipedia:Categorizing articles about people]] guideline outlines the rules on these categories in more detail.

Note that some categories can be non-diffusing on some parents, and diffusing on others. For example, {{cl|British women novelists}} is a non-diffusing subcategory of {{cl|British novelists}}, but it is a diffusing subcategory of {{cl|Women novelists by nationality}}.

{{Anchor|Sort order|SORTKEY}}