If it exists, there is porn of it — no exceptions.
Allegedly originating from and popularized by the imageboard 4chan, this was the very first near-universally agreed-upon Rule of the Internet. It is so well founded and documented with irrefutable proof that even those with only a cursory awareness of the Internet are aware of this rule even if they don't know it has a name.
While the original architects of the Internet had grandiose goals of research and data sharing, we all know what Joe Everyman is going to use it for: pornography! Now, it's not that everyone online is just looking for pornography; it's just that it's very very easy to come to. Even if you're not looking for it! Don't believe us? Do a Bing image search (filters off) of, well... pretty much anything. Sometimes even with the filters on. Odds are pretty good that the results will include something Not Safe for Work.
The key reason the scope of it is so wide and bizarre lies in what some have come to call Rule 36: "If you've thought of it, then there's somebody out there with a fetish for it." (And incidentally, by "it" we mean "anything that exists in the world.") Even TV Tropes.
There's also Rule 35, basically a guarantee that Rule 34 will remain true because "if there is no porn of it, it will be made." This basically means that if you notice you can't find porn of something, and point it out, somebody will be happy to draw/write/find it for you in pretty short order, if only to maintain the Rule 34.
Finally, Rule 35 also has a quantum interpretation. "Referring to a type of previously non-existent porn will cause online porn of that type to come into being retroactively." Nobody takes this seriously, so far as we know.
You might wish to keep a bottle of Brain Bleach handy while proving (or attempting to disprove) Rule 34. Don't say we didn't warn you. And if you DO go hunting to prove this rule false, say good-bye to your childhood first...
An Older Than Dirt trope as decades before the Internet was even conceived, publications known as "Tijuana bibles" circulated widely, featuring X-rated (and copyright-infringing) cartoons showcasing a mix of popular cartoon characters and movie stars of the day doing the Rule 34 thing.
Compare Sexy Whatever Outfit (a non-pornographic Sister Trope), Memetic Molester, Perverse Sexual Lust, the fetish fuel wiki, Rule 34 – Creator Reactions (a list of creator reactions to this applying to their work). See also Rule 63, which is both often confused with this and constantly overlapped.
Not to be confused with these other rule 34's (though overlap with this rule is guaranteed):
- US federal courts' Rule 34.
- Rule 34 of the Evil Overlord List (dealing with the trope Scaled Up).
- The 34th Rule, a Star Trek: Deep Space Nine book (the 34th Rule of Acquisition reads "War is good for business").
- Wolfram's Rule 34.
- Charles Stross' novel Rule34 (although that title is a deliberate Shout-Out).
- Lyons Township High School's rule book #34: No Pornography (which may or may not be a huge coincidence).
- Rule XXXIV of the Death Note (Death Note Owners are protected from other Gods of Death).
- Route 34 (Which happens to be the first place ever in any Pokémon game you could breed your 'mons, and Ditto, the ultimate Pokémon breeding machine, can be caught here.)
- Leroy Jethro Gibbs's Rule #34. While apparently not yet specified by the show, Gibbs has at least 69 rules, so a rule 34 undoubtedly exists.
No examples, please. Not only would such a list cover potentially every work ever, but we're also not interested in being the Net's "How To Find Rule 34 Stuff" — you will need to look elsewhere if you want to find that kind of thing. At most, you can see our pages for The Rule of First Adopters and Parallel Porn Titles to witness how the rule applied even before the Web was invented.
And go to The Internet Is for Porn for in universe examples.
Yes. There is porn of this article as well. No exceptions!