1. First off, with the exception of Barney, every member of both friend groups lives in the main apartment at some point.
2. Ross and Chandler meet in college and become BFFs, and so do Ted and Marshall.
3. The girl Ted dumps on her birthday is also dumped on her birthday by Mike when he gets back together with Phoebe.
4. Both gangs have no idea what Chandler or Barney actually do for a living.
5. In a flashback, Joey attempts to seduce Monica by getting completely naked. Meanwhile, "The Naked Man" is a seduction move Barney and Ted use in How I Met Your Mother.
6. Ross and Monica's mother and Lily's grandmother are played by the same actor.
7. Joey practices his lines for a new role with a pineapple and "The Pineapple Incident" is a long-running gag on How I Met Your Mother.
8. Both Ted and Ross always feel the need to correct their friends' grammar.
9. Phoebe and Barney both frequently use fake names.
10. Both Chandler and Monica and Marshall and Lily are married by one of their best friends.
11. Phoebe refers to Ross and Rachel as each other's "lobsters." On How I Met Your Mother, Lily tells Robin that Barney is like her "lobster" — a clear nod to the iconic Friends moment.
12. Ted and Stella's wedding is ruined when Stella realizes she has feelings for her ex and Ross and Emily's wedding is ruined because Ross has lingering feelings for Rachel.
13. Both Phoebe and Ted learn that somebody is using their name in a series of porn films.
14. In Friends, Rachel dates a guy who looks exactly like Ross, named Russ. And in How I Met Your Mother, each of the characters has a doppelgänger that looks like them that they come across throughout the series.
15. Rachel keeps her pregnancy a secret at Monica and Chandler's wedding, while Lily keeps her pregnancy a secret at Punchy's wedding.
16. Phoebe and Barney both learn that one of their birth parents posed as a family friend when they were younger.
17. Both Chandler and Marshall are terrible at taking a decent photo.
18. Ross and Ted both end up becoming professors after leaving their original jobs.
19. Also, they both date one of their students.
20. Marshall makes up songs for basically everything he does, while Chandler tries to get a job in advertising where he comes up with catchphrases.
21. Both Rachel and Robin are notoriously bad cooks.
22. Ted and Robin's exes end up together, while Ross and Rachel's exes are implied to have gotten together too.
23. Both friend groups have some strange pets: on Friends they have Marcel the Monkey, and The Chick and The Duck. On How I Met Your Mother, they have a goat.
24. In Friends, Phoebe refers to her doctor as "Doogie," in reference to a character played by Neil Patrick Harris.
