- When we edit some article, we usually place it in some category.
- When we launch a category (See: Help:Category Launching), it lists a directory of all the articles that have been designated in that category.
- We can designate a category as being within a larger category.
- When we link to that larger category, we see that in addition to the directory of all articles designated in that larger category, there is also a list of sub-categories that have been designated.
- For example CLP and RPG are programming languages with articles on different aspects and versions of those languages.
- Thus each programming language can be both a category for articles about that language and a sub-category of programming.