To start the race swimmers dive from the starting blocks. Races are two lengths of the pool for the 100m Butterfly. The turn in the Butterfly events is an open turn, with the International Swimming Federation (FINA) rules stating that both hands must touch the end of the pool simultaneously.
The basics
Competition format
All races start with heats. The top 16 swimmers from the heats progress to the semi-finals, and the top eight then advance to the final. As in the heats, the swimmers’ seedings are used to decide which semi-final they swim in, and which lane they are allocated.
If there is a tie for the last qualifying position, a swim-off decides which athlete progresses.
Find out more about the men's and women's 100m Butterfly competitions at the Olympic Games on the International Olympic Committee website.
