Wikipedia:Tip of the day/October 24
The encyclopedia proper is the main namespace and contains all of Wikipedia's encyclopedia articles. All of the other namespaces have prefixes, which must be included (followed by a colon) at the beginning of links to pages in those namespaces in order for those links to work. Otherwise, the links will point to the main namespace, which has no prefix. The namespace prefixes are: Talk:, User:, User talk:, Wikipedia:, Wikipedia talk:, MediaWiki:, MediaWiki talk:, Template:, Template talk:, Help:, Help talk:, Category:, Category talk:, Portal:, Portal talk:, File:, File talk:, Book:, Book talk:, Media:, and Special:. Counting those and the main namespace makes 22.
In any given namespace there can only be one page with a particular name.