Anyone who's been exposed to the 1D fandom for more than 2.6 seconds has encountered the name "Larry Stylinson." I mentioned the ship names of the confirmed couples in One Direction in an earlier post - Louis and Eleanor are "Elounor," Liam and Danielle are "Payzer," and Zayn and Perrie are "Zerrie," but many fans believe there's an unconfirmed and not-so-secret couple in the group - "Larry Stylinson." That's Louis Tomlinson and Harry Styles.
Some Directioners like to ship all the boys in every possible configuration (Ziam, Narry, Ziall, etc.) but the biggest and most popular, and simultaneously most hated and most controversial ship in the group is Larry.
Loads of evidence has been given for the relationship by its supporters - corresponding tattoos, secret hand signals, etc. Frequently the boys even seem to confirm it themselves.
For some pretty compelling evidence (mixed in with some pretty anecdotal and questionable evidence as well) see this rather lengthy video.
Is Larry real? I won't be the one to say, but they'd certainly be a gorgeous couple, wouldn't they?