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 referred to as gears in the bike world, these bicycle gears are technically sprockets since they drive or are driven by a chain, and are not driven by one another.

Modern front and rear derailleurs typically consist of a moveable chain-guide that is operated remotely by a Bowden cable attached to a shifter mounted on the down tube, handlebar stem, or handlebar. When a rider operates the lever while pedalling, the change in cable tension moves the chain-guide from side to side, "derailing" the chain onto different sprockets.

For more information about the choice of particular gear ratios and sprocket sizes, see Bicycle gearing.

Various derailleur systems were designed and built in the late 1800s. The French bicycle tourist, writer and cycling promoter Paul de Vivie (1853–1930), who wrote under the name Velocio, invented a two speed rear derailleur in 1905 which he used on forays into the Alps. Some early designs used rods to move the chain onto various gears. 1928 saw the introduction of the "Super Champion Gear" (or "Osgear") from the company founded by champion cyclist Oscar Egg, as well as the Vittoria Margherita; both employed chainstay mounted 'paddles' and single lever chain tensioners mounted near or on the downtube. However, these systems, along with the rod-operated Campagnolo Cambio Corsa were eventually superseded by parallelogram derailleurs.

Steven J. "Steve" Morse (born 28 July 1954) is an American guitarist and composer, best known as the founder of the Dixie Dregs, and the guitar player in Deep Purple since 1994. Morse's career has encompassed rock, country, funk, jazz, classical, and fusion of these musical genres. In addition to a thriving solo career, he enjoyed a brief stint with Kansas in the mid 80s.