Editing Incubator escapee wiki:Categorization (section)

=== 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}}