Roller derby is a contact sport played by two teams of five members roller skating
in the same direction around a track. Game play consists of a series of
short matchups ("jams") in which both teams designate a scoring player
(the "jammer") who scores points by lapping members of the opposing
team. The teams attempt to assist their own jammer whilst hindering the
opposing jammer—in effect, playing both offense and defense
simultaneously. Roller derby is played by approximately 1,250 amateur leagues worldwide, nearly half of them outside the U.S.