If it exists, there is porn of it — no exceptions.
Allegedly originating from and popularized by 4chan (though it actually predates that site), 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 very rudimentary knowledge 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.
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...
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 infabillity of Rule 34. (And all that mental scarring you'll suffer in the process of proving Rule 34 false will amount to nothing).
As hard as it is to believe, this trope is Older Than Dirt; 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. And it really is Older Than Dirt: depictions of sex have been around almost as long as sex has been around. (Or, at least since early humans developed any semblance of abstract or symbolic thought concerning sex.) While it was originally restricted to drawings and paintings, this has become even more prolific in the last decade or so thanks to art/3D programs such as Adobe Photoshop and Source Filmmaker, especially towards video game characters for the latter. This sometimes reaches the point where a work of fiction will become Best Known for the Fanservice not just for having a sexy character in it, but also for the lewd fanart of such.
When it comes to the creators themselves, their reactions to Rule 34 varies greatly from country to country. In Japan, artists are proud of being 34ed (since it proves they exist...oh, wait.) In the USA, it frequently ends in litigation... with predictable results. In Germany, Rule 34 is known but factually nonexistent and limited to extremely well-known stuff note , so no rules can be given. Also, be advised: In Britain at least, viewing sexualised images of characters depicted as under 16 is treated as a child sex offence under the grounds of "possession of pseudo-images of children". note . While actual prosecutions based on this charge alone have been few, it remains a real consideration in the UK (and is also a hazard for distributors and consumers of legitimate and defensible anime, manga and especially hentai).
Compare Sexy Whatever Outfit (a more tamer Sister Trope), Memetic Molester, Perverse Sexual Lust, the fetish fuel wikinote , 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.
Oh! And what about those visual novels that once had pornographic content, but have removed and forsaken them later on? Well, Rule 34 not only brought them back, but also caused many new pornographic fanarts of the characters. In a way, those NSFW fanarts can be seen as the continuation of those H-scenes. For ecchi anime and manga which do have sexually suggestive scenes, albeit with the clothes still on in some cases, no problem! Rule 34 artists would edit them to make it nude for you.
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. When we said "no exceptions," we meant it. There's porn of everything and the kitchen sink...