The butterfly (colloquially among swimmers known as fly) is a swimming stroke swum on the breast, with both arms moving simultaneously, accompanied by the butterfly kick (also known as the "dolphin kick"). While other styles like the breaststroke, front crawl, or backstroke can be swum adequately by beginners, the butterfly is a more difficult stroke that requires good technique as well as strong muscles. It is the newest swimming style swum in competition, first swum in 1933 and originating out of breaststroke.

Speed and ergonomics

The peak speed of the butterfly is faster than that of the front crawl, due to the synchronous pull/push with both arms. Yet since speed drops significantly during the recovery phase, it is overall slightly slower than front crawl, especially over longer distances.

The breaststroke, backstroke, and front crawl can all be swum easily even if the swimmer's technique is flawed. The butterfly, however, is unforgiving of mistakes in style; it is very difficult to overcome a poor butterfly technique with brute strength. Many swimmers and coaches consider it the most difficult swimming style. The main difficulty for beginners is the synchronous over-water recovery, especially when combined with breathing, since both arms, the head, shoulders and part of the chest have to be lifted out of the water for these tasks. Once efficient technique has been developed, it becomes a smooth, fast stroke.