Other forms: derailleurs
The part of your bicycle consisting of a chain that moves from one toothed wheel to another when you shift gears is called a derailleur.
The French dérailleur, first used around 1930, was inspired by the way a train derails from its tracks, the wheels slipping off to one side. On a bike, this happens on purpose, part of the process of shifting from one gear into another: The chain "derails" off one sprocket and onto another. Before the invention of this device, cyclists had to change their wheels in order to climb steep hills. Most modern bicycles have two derailleurs, in front and back.