Double cylinder is a type of lock mechanism characterised by the requirement of a key to engage the lock from both the exterior and interior sides. Unlike single cylinder locks, which feature a keyed entry on one side and a thumb turn on the other, double cylinder locks demand key operation for both locking and unlocking on either side, enhancing security by preventing entry or exit without the proper key. This design mitigates the risk associated with windows adjacent to doors, where potential intruders could break the glass, reach in, and disengage a single cylinder lock from the inside.
The security provided by double cylinder locks comes with considerations for emergency egress. Codes and standards in many regions require that egress from buildings be possible without the need for a key to ensure safety in cases of fire or other emergencies. Owners and installers of such locks must balance the increased security against potential delays in emergency evacuations. Double cylinder locks find prevalent use in commercial settings where unauthorised access poses significant risks, such as in retail stores and offices with valuable equipment. In residential applications, their use is more restricted and often subject to local safety regulations. These locks are compatible with a variety of door materials and styles, reinforcing their versatility in security applications.