"there are certain codes that guys live by. i've mentioned many of them in this space over the years -- stuff like "if you're sharing a bed with someone in vegas, make sure you remain at least two feet apart at all times," and "if your buddy's team loses an especially tough game, you can't call him to make fun of him under any circumstances." but there are three codes that supercede all others. here they are:

(1) you can't be attracted to your buddy's sister.
(2) you can't be attracted to your buddy's girlfriend.
(3) you can't be attracted to your buddy's ex-girlfriend if he had genuine feelings for her."

the first season of "friends" was the closest anyone ever came to capturing generation x on tv. the characters discussed misunderstandings from "three's company," made jokes about "joanie loves chachi," even hummed the theme from "the odd couple." they were constantly fending off nitpicking parents and nosy neighbors. they busted each other's chops, made constant wisecracks, ripped each other's latest boyfriends and girlfriends. some of them had a little money, others were pretty much broke, and there was always tension between the haves and the have-nots. and they were always happiest just sitting around and doing nothing.

the first sign of trouble happened at the start of season two, when rachel realized that ross was in love with her, planned on reciprocating...and he came back from china with an astonishingly average-looking new girlfriend.
-bill simmons, talking about "friends"-