Rule 34 is a commonly quoted internet "rule" originating from 4chan which states, in essence, that if something exists, there is porn for it. (In this case, "something" can be defined as any person, place, organism, inanimate object, philosophical concept, or anything else that can conceivably be sexualized, as well as several things that can't.) Even the most bizarre and unspeakably deviant fetishes known to humanity are represented in Rule 34, the details of which are too horrifying even by PPC standards to cover.
An addendum called Rule 35 adds that if porn of something does not exist, someone will inevitably make it, usually as the result of a request from someone else who is perverted enough to look for it in the first place.
Due to the pervasiveness of the Narrative Laws, these rules should probably not be invoked or even mentioned in HQ or, indeed, anywhere else. They are often witnessed in action by the Department of Bad Slash.
Rule 34, b. ext.Edit
The extension of Rule 34 is that if there are two characters (or objects, or animals, or anything really), there is a pairing, and someone ships it. More or less a fan-based derivative, it nevertheless stands as a subsection of the original rule quite proudly, and would like it if you wouldn't make fun of it.