Like gears, chain and sprocket drives are used to transmit rotary motion from one place to another and just like gears the ratio of speed between two sprockest is the same a the ratio of the number of teeth. In the example below there are two wheels, one with twelve teeth one with twenty. 12:20 is 3:5

Unlike gears however, the direction of rotation for any wheels within the chain is the same Any sprocket on the outside pressing up against the chain will rotate the other way.
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Also unlike gears, the wheels do not touch, they can be as far apart as the chain allows. Any number of sprockets can be driven by a a single chain. In complex mechanisms, such as car engines, the chain often loops round several different sprockets driving them and keeping them synchronised.

Rather than using a chain, which can be rather noisy, a toothed rubber belt is often used in car engines to drive the camshaft and valves.