The interesting thing about this Neck Collar is the unique design.  Someone has cleverly modified the darby handcuffs/leg irons and enlarged them enough to accomodate an 18 inch (46cms) neck!  The collar works exactly the same way as the darby design, consequently there is no possibility of the collar being over tightened. There is a strong ring to enable attachment of a lead chain or other device.  Comes supplied with one key.