This refers to a system in which a set of locks are keyed different, and yet can all be operated using a single key as well. An example would be a set of apartments. Each tenant has a key to his/her own unit which does not work the others. The landlord, however, carries a single master key that operates all units. Office buildings, retail stores, schools, hospitals and churches all utilize master keys systems. Only a professional locksmith can design, implement and maintain a masterkey system properly! See also change key and submaster key.