Derailleur gears 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. "Derailleur" is a French word, spelled "dérailleur" in French, derived from the derailment of a train from its tracks. Its first recorded use was 1930. At least one notable bicycle industry author has attempted to promote the anglicized spelling of "derailer". As of 2013, however, neither Merriam-Webster nor the Oxford English Dictionary has an entry for "derailer". Various derailleur systems were
Derailleur gears clamp around the seat tube, below the front derailleur, and at least one attaches to the front derailleur mount. Derailleurs require the chain to be in movement in order to shift from one ring or sprocket to another. This usually requires the rider to be pedalling, but some systems have been developed with the freewheel in the crankset so that the chain moves even when the rider is not pedalling. The Shimano FFS (Front Freewheel System) circa 1980 was the most widespread such system. Chain-drive systems such as the derailleur systems work best if the chain is aligned with the