Why do we use an X?

"Why do we use an X?", you say. Well, this is prolly at most FAQ pages, and i'm not positive it's true, but I beleive it is. Back in the 80's, a lot of punk shows were at bars or other such establishments (just like now). And then, the under-age kids had to be marked with an X when they went in so it'd be easier to keep track of who can or cannot buy alcohol at the bar. With the straight-edge movement getting started in the early 80's (thanx to the drummer of Minor Threat), it started catching to mark yourself with an X if you were straight, no matter what your age. So, even a straight-edge show-goer was 21 or older, he(or she) would slam an X on their hand to show they abstain from such poisens.
And that's my story!