the rule which state that you are able to date someone 2 years older or younger (2 knuckles from the middle both ways) without consiquence
Dude1: yo last night i hooked up with this hot chick from collage
Dude2: u mean the blonde one? shes 2 years older then you!
Dude1: knuckle rule
Dude2:good point, nice job last night