friend rule 

The rule that you shouldn't get romantically involved with someone a close friend has been romantically involved with, at least without asking first. The reason being that whatever issues exist(ed) in the romantic relationship tend to become issues for the friendship as well.

"Did you hear that Mike is sleeping with Sara?"
"Isn't she your ex? Total violation of the friend rule, man."

friend rule by Memeslayer October 6, 2006

Best Friends Rule 

Having the fixation on doing equal sexual favors for a pair of best friends.

Brandon - Hey Libby wanna hang out?
Libby - Sure, only if Matt comes though.
Brandon - Best friends rule?
Libby - Oh yeah, all night ;)

Best Friends Rule by MattH1008 January 17, 2009