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A game of roller derby is called a bout. A bout is divided into plays, called jams, which may last up to two minutes. To start a jam, each team sends five skaters onto the track: four blockers and one jammer.
One blocker from each team wears a stripe on their helmet; this is the pivot. The blockers from both teams, together, form the pack.
The jammer, who wears a star on their helmet, starts behind the pack and is the only one who can score points. The jam starts when the whistle blows! Blockers play offense and defense simultaneously, attempting to assist their jammer through the pack while preventing the opposing jammer from getting through. The first jammer to pass all of both teams’ blockers legally and in-bounds is the lead jammer, which allows them to call off the jam. Both jammers begin to score points on their second and subsequent passes through the pack.
Until the end of the jam (either at two minutes or until the lead jammer calls the jam), the jammers score a point for each opposing skater they pass, including those in the penalty box.
The full rule set can be found at http://www.wftda.com/rules. And you can watch some great derby online at http://www.wftda.tv/!