play by WFTDA rules, while members of the former Canadian Women's Roller Derby Association were encouraged to join the WFTDA. Basics of play Roller derby is played in two periods of 30 minutes. Two teams of up to 15 players each field up to five members for episodes called "jams". Jams last two minutes unless called off prematurely. Each team designates a scoring player (the "jammer"); the other four members are "blockers". One blocker can be designated as a "pivot"—a blocker who is allowed to become a jammer in the course of play. The next jam may involve different players of the 15 roster players, and different selections for jammer and pivot. During each jam, players skate counterclockwise on a circuit track. Points are scored only by a team's jammer. After breaking through the pack and skating one lap to begin another "trip" through the pack, the jammer scores one point for passing any opposing blocker. The rules describe an "earned" pass; notably, the jammer must be in-bounds and upright. The jammer's first earned pass scores a point for passing that blocker and a point for each opponent blocker not on the track (for instance, serving a penalty, or when the opposition did not field five players for the jam). If the jammer passes the entire pack, it is a four-point scoring trip, commonly called a "grand slam". Each team's blockers use body contact, changing positions, and other tactics to help their jammer score while hindering the opposing team's jammer. Jams Play begins by blockers lining up on the track anywhere between the "jammer line" and the "pivot line" 30 feet in front. The jammers start behind the jammer line. Jams begin on a single short whistle blast, upon which both jammers and blockers may begin engaging immediately. The pack is the largest single group of blockers containing members of both teams skating in proximity, arranged such that each player is within 10 feet of the next. Blockers must maintain the pack, but can skate freely within 20 feet behind and ahead of it, an area known as the "engagement zone". The first jammer to break through the pack earns the status of "lead jammer". A designated referee blows the whistle twice and continually points at the jammer to confer lead jammer status, which lets that jammer stop the jam at any time by repeatedly placing hands on hips. Lead