Derailleur gears (a variable-ratio transmission system) is falsely thought to not have variations, and they have a cage that holds two pulleys which are known as the  jockey pulley
Derailleur gears are a variable-ratio transmission system commonly used on bicycles, consisting of a chain, multiple sprockets of different sizes, and a mechanism to move the chain from one sprocket to another. Although variations exist, as noted below, most rear derailleurs have several components in common. They have a cage that holds two pulleys that guide the chain in an S-shaped pattern. 
