Handcuffs are restraint devices designed to secure an individual's wrists in proximity to each other. They comprise two parts, linked together by a chain, a hinge, or rigid bar. Each cuff has a rotating arm which engages with a ratchet that prevents it from being opened once closed around a person's wrist. Without a key, handcuffs cannot be removed without specialist knowledge, and a handcuffed person cannot move their wrists more than a few centimetres or inches apart, making many tasks difficult or impossible.