From Wayne's Dusty Box of Words

To add a page to a category, add this text to the bottom of the page: [[Category:Name]] A page can be part of more than one category.

A category page is created by creating a page in the Category: namespace; just add "Category:" before the page title.

Link to a category: [[:Category:Help|Help category]] → Help category

Create a hierarchy by adding a (sub)category to a category page.

Category:Mathematical logic hierarchies
In [[mathematical logic]], these [[hierarchy (mathematics)|hierarchies]] classify various objects, such as real numbers (equivalently, sets of natural numbers) and sets of real numbers, based on various definability and complexity criteria.

This category contains articles about these hierarchies as well as theorems and terms relating to them.

[[Category:Mathematical logic|Hierarchies]]