Roller derby is a roller skating contact sport played by two teams of fifteen members. Game play consists of a series of short scrimmages (jams) in which both teams designate a jammer (who uniquely wears a star on the helmet) and four blockers to skate counter-clockwise around a track. The jammer scores points by lapping members of the opposing team. The teams attempt to hinder the opposing jammer while assisting their own jammer—in effect, playing both offense and defense simultaneously.