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.
Derailleur is a French word, correctly spelled dérailleur, 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.
She graduated high school I bought her a trailer
In a little park by the side of the road
I could've had the army, I could've had the navy
But no, I had to go for a mobile home
Yeah I guess, I gave it all for you, babe
There wasn't room in that trailer for two, babe
I kept up with my interest, I kept up with my payments
She never said goodbye, I never asked why
Man, we used to dance to Lynyrd Skynyrd
Boy she used to look so good at times
But I guess, I gave it all to you, babe
There's not room in no trailer for two, babe
Well I guess, I gave it all for you, babe