- For a pre-existing category, the article of the same or similar name and (rarely, or) on the same topic should be added to that category. When creating an article one should, only if appropriate (especially horizontally), create a category of the same or similar name on the same topic.
- Topical category names should be singular. Examples: "Law", "Civilization"
- Note that there are a growing number of category pairings, where the singular category (of related topics) and plural category (of its instances) both exist (for example, Category:Opera and Category:Operas). When categorizing articles, be careful to choose the correct variant.
In turn, Wikipedia:Naming conventions (categories)#Special conventions for lists of items policy states:Category names follow different pluralization conventions, see Wikipedia:Naming conventions (categories).
Some categories have been adding "topics" to the singular to prevent confusion, but that's relatively new and controversial.Note that there are a growing number of category pairings, where the singular category (of related topics) and plural category (of its instances) both exist (for example, Category:Opera and Category:Operas). When categorizing articles, be careful to choose the correct variant.