Master keys systems are carefully planned key operated locking systems that allow a hierarchy of people access into certain doors defined by the commissioner of the system. An example would be where a House builder builds a block of apartments and gives each owner one key. Each individual would be able to open the communal door and their own apartment door, but not anybody else's.
However, the caretaker of the property would have one key that would open multiple communal doors, and also other defined doors. If there was a managing agent for the whole building, they theoretically could have one key that opens all the doors in the building. In addition to the simplicity that masterkey systems provide, they also provide additional security using extra pins within the cylinders to prevent picking.
Masterkey systems are becoming increasingly popular, as commercial building deploy them in relation to their own hierarchy. For example, an MD of the company could have full access to all doors; where as another member of staff would only need access to a limited number of doors. If an incident occurred inside a restricted room, it would eliminate everybody but the key holders. In large cooperation's this can be an extremely time saving operation, not to mention less complex and more secure.