What is the 15-metre rule, and where does it apply?

In freestyle, backstroke and butterfly, some part of the swimmer's body must break the surface throughout the race, except that the swimmer may be completely submerged during the turn and for up to 15 metres after the start and after each turn. By the 15-metre mark the head must have broken the surface.

Near the finish: in these three strokes the swimmer may be completely submerged once part of the head has passed the 5-metre mark before reaching for the finish wall.

Breaststroke has no 15-metre rule. Instead, after the start and each turn, the swimmer may take one arm stroke back to the legs while submerged, and the head must break the surface before the hands turn inward at the widest part of the second stroke. After that, part of the head must break the surface in every stroke cycle.