At punk rock shows, it became common practice to mark an X on the hands of under-aged concert goers to ensure that the bouncers would recognize a minor attempting to drink alcohol. Some people interpret this as a symbol of Ian MacKaye's "don't smoke, don't drink, don't fuck" ethos. Some interpret the three Xs as representing "Body", "Mind", and "Soul"—although three Xs have also been used as an abbreviation for hardcore punk in general.
Many adopters of the "straight edge" lifestyle voluntarily marked their hands in the same way to show their commitment to refusing alcohol. Also widespread is the tattooing of the X symbol on other parts of the body or wearing it on clothing, pins, et cetera. Three Xs (XXX) have their origin in artwork created by Minor Threat's drummer Jeff Nelson in which he replaced the three stars in the band's hometown Washington D.C. flag with Xs.
The X is considered both a mark of negation and a mark of identity. Attaching the X to one's name or band name is common practice for straight edgers. For example, 'John Smith' would become 'johnXsmith', or 'xjohnx'. "Straight edge" is sometimes abbreviated sXe (S.E. plus an X) following much the same logic and hardcore is sometimes abbreviated "hXc".