Roller derby is played by two teams simultaneously skating counter clockwise on a circuit track. Each team fields five players per “jam”, both teams designate a scoring player (the “jammer”); the other four members are “blockers.” One blocker can be designated as a “pivot”—they set the pace for game play. The jammer wears a helmet cover bearing two stars; the pivot wears a striped cover; the remaining members’ helmets are uncovered. The bout is played in two periods of 30 minutes. Point scoring occurs during “jams”: plays that last up to two minutes. During a jam, points are scored when a jammer on a scoring pass (every pass a jammer makes through the pack after the initial pass) laps members of the opposing team. Each team’s blockers use body contact, changing positions, and other tactics to assist its jammer to score while hindering the opposing team’s jammer. Certain types of blocks and other play are violations; referees call penalties and require violators to serve time in a penalty box.