A common "game" where if someone stands up, they say "17's", which saves their seat for 17 minutes. Often compared to "fives", which stole the idea from 17s, but it saves the seat for only 5 minutes. If someone steals the seat before 17 minutes are up, he is subject to beatings until he leaves the seat.
Person 1: Hey, I'm going to go buy some milk, 17's.
Person 2: (after P1 leaves, before 17 minutes) Haha I'm gonna steal his seat.
Person 3: Person 1 is gonna beat the crap out of you if you don't leave.