Handcuffs are restraint devices designed to secure an individual's wrists close together. They comprise two parts, linked together by a chain, a hinge, or rigid bar. Each half has a rotating arm which engages with a ratchet that prevents it from being opened once closed around a person's wrist. Without the key, the handcuffs cannot be removed and so the handcuffed person is unable to move his or her wrists more than a few centimetres or inches apart, making many tasks difficult or impossible. This is usually done to prevent suspected criminals from escaping police custody.