1. The show is a real family affair. As most people know, Mac and Dee are married in real life, as are Charlie and The Waitress. On top of that, Caylee, the woman Dennis "re-D.E.N.N.I.S.es" in "The D.E.N.N.I.S. System" is actually his wife, Jill Latiano.
2. Day met Mary Elizabeth Ellis (The Waitress) in 2004 when they played incestuous twins on Reno 911.
3. The reason Glenn Howerton's character is named Dennis, while Charlie Day and Rob McElhenney's characters are named after themselves, is because Howerton wanted to distance himself from "sociopath" Dennis as much as possible. Fair enough, tbh.
4. There is a hidden audio message from McElhenney, Howerton, and Day over the end credits of every episode of Always Sunny. They are played backwards and are generally pretty weird.
5. There was a one-off TV short spin-off back in 2007 called Danny DeVito and the Contract. The plot is that Charlie Day forgets to read his contract for season three of Always Sunny, and the contract says that he has to perform oral sex on DeVito.
6. Rob McElhenney has appeared in a few other notable TV shows. He played Aldo in two episodes of Lost, Lou Tookers in The Mindy Project, and here he is in a episode of Law and Order from way back in 1997.
7. Glenn Howerton has also appeared in The Mindy Project, as Cliff Gilbert, and he's done voice work on shows like Family Guy and American Dad!.
9. Charlie Day has also done some pretty notable voice work – he played Benny the spaceman in The Lego Movie and Art in Monsters University. He also acted alongside Jennifer Aniston in Horrible Bosses.
10. Mac actually has two gay cousins, Country Mac, and Brett, who appears in the very first episode when The Gang accidentally turn Paddy's into a gay bar.
11. Carl Weezer from Jimmy Neutron bears an almost uncanny resemblance to young Frank Reynolds, AKA "Froggy".
12. Dicktowel.com is a real website, and you can buy real life dick towels from it.
14. Charlie Day has actually tried rum ham. He says it's "disgusting", which is really disappointing.
15. In "Dennis and Dee Get a New Dad", when Charlie and Mac are trying to prove how tough they are to Mac's dad, you can see Charlie giving himself a tattoo in the background of one scene. It is supposed to say "Bad News", but he gets interrupted by Frank, so it only says "Bad New". That "Bad New" tattoo has remained on Charlie's arm for every other episode since.
16. "Mac and Dennis Break Up" in season five is the first episode not to feature any scenes in Paddy's.
17. This is how straight Mac is, according to one of his chapters in The Gang's self-help book, "The 7 Secrets of Awakening the Highly Effective Four-Hour Giant, Today".
18. Mac's childhood home in "Mac Bangs Dennis' Mom" is actually Rob McElhenney's aunt and uncle's house in Philadelphia. There is a white house to the right of it that can also be seen in the episode, which is the actual house McElhenney grew up in.
19. In the same episode there is a waitress who Frank fancies. Weirdly, that waitress is actually played by his real-life daughter, Lucy DeVito.
20. The shows original pilot was a short film version of season one episode "Charlie Has Cancer". According to Howerton, the show was also originally called "Jerks", and was about a group of struggling actors rather than a gang who run a bar.
21. In the original pilot Dee was played by another woman – Rob McEnhenney's then-girlfriend Jordan Reid. They broke up and Kaitlin Olson ended up getting the role (and Rob). Bridesmaids star Kristin Wiig also auditioned for the role.
22. The real exterior of Paddy's Pub isn't actually in Philly – it's the Starkman building in Los Angeles.
23. There is a deleted scene from "The Nightman Cometh" in which Mac has his own song. You can find it at the end of this video.
24. The Gang have actually performed musical episode "The Nightman Cometh" live. Danny DeVito's wife Rhea Perlman played the role of Gladys in these live shows.
25. Harry Potter himself, Daniel Radcliffe, is a big fan of the show.
26. And Sophie Turner (Sansa Stark in Game of Thrones) is also a fan.
27. That's not the only link the show has to Game of Thrones – the episode "Flowers for Charlie", (based on the book "Flowers for Algernon" by Daniel Keyes) where Charlie takes pills which make him believe he has become super intelligent, was co-written by Thrones writers D. B. Weiss and David Benioff.
28. Reddit user baconcowboy noticed something very Always Sunny about the episode "The Gang Gets Trapped" – it looks like they have bought an Indiana Jones costume, but then not been able to decide who gets to wear it.
29. As most fans know, Rob McElhenney deliberately got fat for season seven to mock how the characters always seem to get steadily better looking in other sitcoms. He wanted the rest of the cast to join him in gaining wait (except DeVito, who he wanted to get really thin), but they all refused. He ate 5,000 calories a day and ended up weighing 212lbs.
30. "Greenman" is inspired by a real person – McElhenney based the idea on a friend of his who always used to go to Philadelphia Eagles games in a bright green morphsuit. He later ended up paying this friend an undisclosed about of money for full rights to the "character".
31. In the final episode of season one, "Charlie Gets Molested", Charlie mentions that he has a sister. It's the only time she has been mentioned in the entire series and she has never appeared on screen.
32. In "Dee Reynolds: Shaping America's Youth" (the episode in which The Gang remake Lethal Weapon 5), one of the kids Dee brings to Paddy's to watch the movie says "I've been to this bar". This is probably a callback to season one's "Underage Drinking: A National Concern", when The Gang decide to start serving underage drinkers.
33. Charlie Day looks so totally different without a beard that it's kinda scary.
34. There's a Russian version of the show called It's Always Sunny In Moscow where The Gang owns a bar called Philadelphia. It's pretty weird.
35. Charlie Day once called the show's opening credits the "cheapest ever". They were shot by The Gang with a handheld camcorder as they drove around Philly one night.
36. In "Sweet Dee Has A Heart Attack", Frank escapes from a mental institution. This is a nod to One Flew Over the Cuckoo's Nest, in which Chief Bloom does the same thing in a very similar manner. Danny DeVito played a minor role in that film.
37. Danny DeVito is a vocal supporter of Bernie Sanders over Hillary Clinton in the race for the Democratic nomination – and it turns out Frank isn't especially keen on Hillary either.
38. There is a fan theory that Frank is the only human in the show, and the rest of The Gang are pets he adopted. Charlie is a rat, Dennis is a cat, Mac is a dog, and Dee is (obviously) a bird. It's pretty out there, but it kiiinda holds up.
39. Season one had pretty poor ratings, so FX demanded that a big name star was added to the cast to keep it alive. Danny DeVito agreed to be on it because his kids loved the show, and he enjoyed being part of it so much that he's stuck around ever since.
40. The Gang have never looked back – the show became the longest running live action comedy series in cable history when it renewed for its 11th and 12th seasons in 2014 – taking it up to 133 episodes.
41. And finally, here's how much everyone's favourite priest turned homeless dog orgy-loving drug dealer, Rickety Cricket, has changed in 10 beautiful years of Always Sunny.