What specific requirement did the Tour de France mandate in 1937 regarding gearing?

That cyclists must use a system providing at least two chainrings on the rear wheel.

The watershed moment that validated the derailleur system over internal hub gears for high-level competition was the regulatory action taken by the Tour de France organizers. Recognizing that single-speed setups artificially limited the sporting challenge of mountainous terrain, race organizers mandated a change for the 1937 race. Specifically, cyclists were required to use equipment that featured at least two different-sized chainrings attached to the rear wheel. This regulation immediately favored the lighter, external derailleur setup, as it provided the necessary mechanical variation more efficiently than the heavier hub systems, thereby forcing rapid advancement in derailleur durability and mass production.