In order to compete, swimmers are required to keep at least a portion of their bodies above water at all times.
During the turn, at the finish, and for a maximum of fifteen meters after the start and each turn, the swimmer is allowed to remain entirely submerged. At that stage, the skull must have burst through.
The breaststroke event does not adhere to the 15-meter guideline, although the freestyle, butterfly, and backstroke events do.
