Rule 63 is an Internet meme that states that, as a rule, "for every given female character, there is a male version of that character" and vice versa. It is one of the "Rules of the Internet" that began in 2006 as a Netiquette guide on 4chan and were eventually expanded upon by including deliberately mocking rules, of which Rule 63 is an example. It began to see general use in fandom communities as a term to refer to both fan-made and official gender-swaps of existing fictional characters.
