This is one variant for a proposed category intersection feature.

Mockups:

An index page before being named: [[Index:Actor::American people::People of African descent]]
The same page after being named: [[Index:African American actors]]
A navigation index using three categories: [[Index:Bridges by country, by date and by type]]

This option does not make a one to one correlation between categories and intersections. Categories defined as intersections get deleted from category space, instead there can be links to the new INDEX namespace and an easy way to navigate through the namespace. Intersection pages can be named, but the name will only be used as a heading at the top of the page, just below the formal name of the page which comes from the primary category names which are intersected. For example, the heading of the intersection page of Category:Albums and Category:Musical groups might be displayed like this:

Index:Albums::Musical Groups

From Wikipedia, the free encyclopedia

This index of "Albums by artist" combines the following categories:[edit]

Albums

Musical groups

The contents of an index page lists all the articles that are in all the intersected categories. There are also links to the subcategories of each primary category, intersected with the remaining primary categories. These can be thought of as sub-indexes of the index.

The name of the page is "Albums by artist". "Albums by artist" is shown whenever the index is linked as a sub-index from another index page. For example, before being named, the link to the index example above is Albums::Musical groups. Once named, "Albums by artist" is used for the name, so links to the index will appear as Albums by artist .

Since index pages are generated automatically using the primary categories and all the subcategories of the primary categories, it is possible to navigate to those pages and continue as far as the structure allows. It is possible to find the intersections of descendents of each primary category at any level, though it may take a few intermediate steps to get there.

Sub-indexes can be shown for intersections of categories having a huge number of subcategories. Like with categories, only the first 200 are shown. A table of contents can make navigation of huge indexes easier. Since all the index pages can be named in English, it is be easy to figure out which letter of the alphabet you would need to find the index you are looking for.

Index pages can be categorized, just as categories can be categorized. There is a new section in category pages to list the indexes that have been put in the category. Links to the index pages appear. For example, Index:Occupations::People by nationality, intersection of Category:People by nationality and Category:Occupations can be categorized in both primary categories. In a sense, the old categorization structure continues as it was, except at some point you now switch from category space to intersection space by choosing an index instead of a subcategory. The difference helps show that the contents of indexes are created by intersecting primary categories.

The editing of intersection space is minimal. The only editing to Index:Occupations::People by nationality is giving it a name, and perhaps some descriptive text, and categorizing it when appropriate. Many intersections will probably remain unedited because their contents are meaningless or not useful. This is much less work than the previously existing system or the first option, because all the pages are automatically generated. There is no need to restrict any users from editing index pages. The renaming of index pages can have the same restrictions that the renaming of articles has. Pages only need to be deleted if the primary categories are renamed or deleted. If so, it can be a speed rename or delete. Since this system uses many less categories than previously, the amount of discussion at CFD should decrease substantially.

There would not be any automatic method to repopulate primary categories, but bots can be created to do the work of repopulating primary categories and depopulating intersection categories. Since the intersection categories would no longer be used, the bots would be remain useful to keep the categories from getting repopulated. Redirects could direct people to intersection pages.