Derailleur Summary
Dérailleurs consist of a chain, multiple sprockets and a mechanism to move the chain from one sprocket to another.

Modern front and rear dérailleurs typically consist of a movable chain-guide that is operated remotely by a Bowden cable attached to a shift lever mounted on the down tube, handlebar stem, or handlebar. When a rider operates the lever while pedaling, the change in cable tension moves the chain-guide from side to side, "derailing" the chain onto different sprockets. To both guide the chain to the selected sprocket and maintain chain tension by taking up any slack caused by changing to a smaller sprocket, the rear dérailleur has two pulleys in a spring-loaded rotating cage, through which the chain rolls in an S-shaped pattern. The pulleys are known as the guide pulley (top) and the tension pulley (bottom). Together they are commonly referred to as the jockey pulleys or wheels. The front dérailleur has a cage that should touch the chain only while shifting between the front chainrings.
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