The Rules of Flat Track Roller Derby

2.3 - Periods

2.3.1 - The period begins when the designated Official blows the jam-starting whistle. The signal will be one short whistle blast.

2.3.2 - There will be a break of at least five minutes between periods.

2.3.3 - The period ends when the last jam reaches its natural conclusion (see Section 2.4 - Jams). This may extend past the point when the period clock reaches 0:00.

2.3.4 - If 30 seconds or fewer remain on the period clock when a jam ends, there will not be another jam started for that period unless a timeout or Official Review is called by one of the teams (see Section 2.6.5). If an Official Timeout needs to be taken with 30 seconds or fewer remaining on the period clock, the remaining time on the period clock will expire after the Official Timeout and the period will end.