The double lock, which prevents ratcheted handcuffs from tightening after the lock is engaged, is now a common feature on the majority of contemporary handcuffs, independent of cuff form. The universal handcuff key is usually rotated in the opposite way that releases the single lock to disengage the double lock. This prevents the cuffs from tightening unintentionally if, for example, someone falls over. The double lock was invented by George H. Dixon in 1872. Before this time, there were several other designs on the market (some with triple and even quadruple locks). Dixon's patent described his invention as a "new and useful improvement in Key-Handcuffs". It can be used on most standard cuff keys and provides additional security against accidental release. Although this patent expired in 1896, many modern handcuffs still include this feature today. Double locking mechanisms are found on most current models of police handcuffs as well as civilian restraints such as KLINK wristlets and BI-LOCKS. These handcuffs use a common key to engage both locks simultaneously so they cannot be separated without removing the key. Some pre-1930s models used a second set of keys to unlock the individual locks instead. These keys were often called "separate" or "distinct" locks and were required by law in some jurisdictions.