The Rules of Flat Track Roller Derby

1.4 - Periods

1.4.1 - The period begins when the designated Official blows the jam-starting whistle.

1.4.2 - The period ends when the last jam reaches its natural conclusion (see Section 1.5 - Jams). This may extend past the point when the period clock reaches 0:00.

1.4.3 - If 30 seconds or fewer remain on the period clock when a jam ends, there will not be another jam started in that period unless a timeout or Official Review is called by one of the teams (see Section 1.7 - Timeouts).

1.4.3.1 - If an Official Timeout is taken with 30 seconds or fewer remaining on the Official Period Clock, it will begin running once again when the designated Official indicates that the Official Timeout is complete.