Wikipedia:Manual of Style/Category pages

Scope and purpose
The purpose of the categorisation system is to enhance navigation of Wikipedia by grouping related pages together. As in everything in Wikipedia, its primary purpose is to aid our readers, and secondary to that (though not inconsequentially), our editors.

The technical process of categorisation (through the adding of a bracketed category name editorially to a page) is covered at Wikipedia:Categorization. This page covers the creation of the category page at that category name through the standard editorial process.

Overview
Generally the contents of a category should be self-explanatory due to the category having a clear unambiguous name. But sometimes, a common-sense guess based on the title of the category isn't enough to figure out whether a page should be listed in the category. So it may be helpful to further clarify the intended scope and purpose of the category, explaining the intended contents of the category, and aid in navigation to topics related to the category's contents.

This can be done through the use of various banner templates such as container category and catdiffuse. And to help navigation, the primary associated article(s) should be linked on the category page (e.g. science for Category:Science). This can be done through the use of Cat main and Category see also. There are other such hatnote templates, see Category:Hatnote templates for category pages.

In addition, some additional explanatory text may be helpful as well – to include a description of the category, indicating what pages it contains, how they should be subcategorized, and so on. But it should be kept concise and directly related to the scope and purpose of the category and not be an attempt to (re-)write an article on the related topic. Categories, as noted above, are designed for navigation and should never be used to replace content. (In other words, we should potentially be able to remove the entire category system and the only loss should be navigation-related and in no way content-related.) Various templates have been developed to make it easier to produce category descriptions; see Category namespace templates.

The desired contents of the category should be described on the category page, similar to how the list selection criteria are described in a stand-alone list. The category description should make direct statements about the criteria by which pages should be selected for inclusion in the category. This description, not the category's name, defines the proper content of the category. Do not leave future editors to guess about what or who should be included from the title of the category. Even if the selection criteria might seem obvious to you, an explicit standard may be helpful to others, especially if they are less familiar with the subject.

The description can also contain links to other pages, in particular to other related categories which do not appear directly as subcategories or parent categories, and to "sister categories" on other projects, such as Commons. Another technique that can be used is described at Classification.

A maximum of 200 category entries are displayed per screen. To make navigating large categories easier, a table of contents can be used on the category page - e.g. Category TOC displays a table of contents (Top, 0–9, A–Z).

Subcategories are split alphabetically along with the articles, which means that the initial screen of a split category may not include all its subcategories. To make all subcategories display on each screen, add a category tree to the text of the category page, as described at the help page under Displaying category trees and page counts.

As with any Wikipedia page, the categories are typically placed at the bottom of the page.

To prevent an excessive amount of white space from appearing on the page, the Portal and commons cat templates when used should be listed before any other content (typically with the Portal template placed first).

Avoid transcluding images on a category page unless you can show how it will enhance and aid navigation.

With only a few exceptions, explanatory notes concerning WikiProjects should be placed on the category talk page and not on the category page itself.

Interlanguage links work for category pages just as they do for articles, and can be used to link to corresponding categories on other language Wikipedias. These links are held at Wikidata.

Example
As an example Category:Science could be laid out as follows:

In this case the explanation (used here for illustration) of what science is would not be necessary since the vast majority of readers understand what it is and the category does not veer from the essence of the topic.