Derailleurs are a variable-ratio transmission system allowing for multiple gearing on a bicycle.
Various derailleur systems were designed and built in the late 19th century but it took until 1937 for the derailleur system to be introduced to the Tour de France, allowing riders to change gears without having to remove wheels. Previously, riders would have to dismount in order to change their wheel from downhill to uphill mode!
Derailleurs did however not become common road racing equipment until the introduction of the cable-shifted derailleur.
In 1949 Campagnolo introduced the Gran Sport, a hugely successful version of the cable-operated parallelogram derailleurs. It was followed by the slant-parallelogram rear derailleur by Suntour – which the remains the current rear derailleur design of choice.
Before the 1990s many manufacturers made derailleurs, including Simplex, Huret, Galli, Mavic, Gipiemme, Zeus, Suntour, and Shimano. Today the three main manufacturers of derailleurs are Shimano (Japan), SRAM (USA) and Campagnolo (Italy), who only make equipment for road and cyclocross.