... was developed in the 1930s as a variation of the breaststroke. It's difficult to do, but a lot faster than the traditional breaststroke.
According to Wikipedia, the "dolphin kick" leg movement broke the rules for the breaststroke, so a few competitors at the Berlin Olympics of 1936 used the butterfly arm movements with the breaststroke leg movements. By 1938, almost every breaststroke swimmer was using this butterfly style. It was considered a variant of the breaststroke until 1952, when it was accepted by FINA as a separate style with its own set of rules. At the 1956 Summer Olympics, the butterfly was swum as a separate event for the first time: over 100 metres for women and 200 metres for men.
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