A locking system is a coordinated arrangement of locks and keys (or electronic media) that controls physical access to buildings, floors, and rooms. Modern locking systems use transponders or smart cards and log access events. This lets you implement differentiated access rights: specific individuals gain access only to defined areas. In an ISMS, the locking system is part of physical security controls per ISO 27001 Annex A 7.1-7.4. A key plan documents who holds which key. Lost keys must be revoked and replaced promptly.