How it unfolded
Butterfly was born in the 1930s out of a loophole in the breaststroke rules. Because the regulations never specified how the arms should recover, swimmers began bringing both arms over the water, and the variant swallowed the breaststroke event whole. FINA eventually split the two strokes apart, and as a result, butterfly became an independent Olympic event for the first time at Melbourne 1956. The winner of the men's 200m butterfly final was America's William Yorzyk, a late bloomer who hadn't swum competitively in high school and only began serious training in college; he became the first-ever Olympic butterfly champion in an Olympic record of 2:19.3. Japan's Takashi Ishimoto took silver in 2:23.8, and Hungary's Gyorgy Tumpek bronze in 2:23.9. In the women's 100m butterfly at the same Games, America's Shelley Mann won to become the first women's butterfly champion. With that, the four competitive strokes, freestyle, backstroke, breaststroke and butterfly, were complete, and the foundation was laid for the individual medley and medley relay.
Why it still matters
Today's competitive swimming programme was fixed in shape by this final. The fourth stroke's independence made the 200m and 400m individual medley and the 4x100m medley relay possible, and opened the door to swimmers like Michael Phelps and Leon Marchand who bundle multiple strokes into a single event. As a case of a technique born from a rules loophole being promoted to a full event, it's also the template for how this sport responds to innovation.