Thumbcuffs are a metal restraining device that lock thumbs in proximity to each other.
Rarely used due to an increased possibility of injury, most commonly by tight cuffs blocking blood circulation, handcuffs are used instead.
Thumbcuffs may also be used as improvised toecuffs by locking the big toes in proximity to each other.
Thumbcuffs with double locks have a lockspring which when engaged, usually using the top of the key, stops the thumbcuff from ratcheting tighter and prevents the subject from tightening them, possibly causing injury. Double locks also make picking the locks more difficult.