A bar with teeth on its face or edge, to work with those of a gearwheel, pinion, or worm, which is to drive or be driven by it.
A bar with teeth on its face or edge, to work with a pawl as a ratchet allowing movement in one direction only, used for example in a handbrake or crossbow.
A device, incorporating a ratchet, used to torture victims by stretching them beyond their natural limits.