### Instruction: Please give me a summary of the current rule set for roller derby.
### Input: Roller derby is played in two periods of 30 minutes.: 4  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.: 5  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.: 7  The next jam may involve different players of the 15 roster players, and different selections for jammer and pivot.: 7 

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.: 33 [note 1] 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".[note 1]

Each team's blockers use body contact, changing positions, and other tactics to help their jammer score while hindering the opposing team's jammer.
### Response: A roller derby bout consists of two 30 minute halves.
Players will be on the track for a series of two minute intervals.Each team will have 5 players on the track at one time. 
Four players from each are designated blockers, and will play both offense and defense at the same time.
Each team will field one jammer who is the main point scoring skater.
Jammers earn points for passing blockers from the opposing team.