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Handcuffs are restraint devices designed to secure an individual's wrists close together. They comprise two halves, linked together by a chain or hinge. Each half has a rotating part which engages with a ratchet which is closed around a person's wrist. Without the key, the person cannot 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.