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 in the case of rigid cuffs, a 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 unlocked and so the restrained person is unable to move their wrists more than a few centimetres/inches apart, making many tasks difficult or impossible. This is usually done to prevent suspected criminals from escaping police custody.