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.