Rule 63

Refers to one of the Rules of the Internet originally coined as a 4chan meme which essentially states: "For any given male character, there is a female version of that character." In fandom, this rule is the reason for the prevalence of the Genderswap trope, be it the 'sudden-sex-change' or 'always-a-girl' variation on the theme. It also applies to the opposite of female characters rewritten/redrawn as males too.

Hetalia.