Derailer - AbsoluteAstronomy.com

A Derailer may be

A Derail (railroad)
Derail (railroad)
A derail or derailer is a device used to prevent fouling of a track by unauthorized movements of trains or unattended rolling stock. It works by derailing the equipment as it rolls over or through the derail....

A Derailleur A device for changing the gear ratio
Gear ratio
The gear ratio of a gear train is the ratio of the angular velocity of the input gear to the angular velocity of the output gear, also known as the speed ratio of the gear train. The gear ratio can be computed directly from the numbers of teeth of the various gears that engage to form the gear...

of a bicycle.

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