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Ratchet refers to a tool or mechanical device that allows continuous rotary or linear motion in the opposite direction.

A ratchet is composed of a round gear or a linear rack with teeth, spring-loaded finger called a pawl that engages the teeth.

The teeth are uniform but asymmetrical with each tooth having a moderate slope on edge and a much steeper slope on the other side of its edge.

Because a rachet can only stop backward motion at discrete points, a rachet does allow a limited amount of backward motion. The backward motion is limited to its maximum distance equal to the spacing between the teeth.