Dérailleur A mechanism for moving the chain from one sprocket to another to change gears on a multi-speed bicycle. A typical derailer consists of a parallelogram which moves a cage. In the case of a rear derailer, the cage will have two chain pulleys, a jockey pulley and a tension pulley. Different derailers have different capacities to handle different gear ranges. Also commonly spelled in the French style " Derailleur." An extensive article on Derailer Adjustment is available on this site. Britain's Cyclists' Touring Club Website has a good article on derailer gearing.