A key to operate a number of locks which all differ having their own servant keys.
A master key system involves two or more locks operating individually from each other using their own keys. Each key will open the lock it was issued with but will not operate any other locks within the master key system. A master key is supplied with each master key system and this key will open all locks fitted under that system.
A typical situation where a master key system would be used is in a hotel. Each room in the hotel is locked with its own individual key and this key cannot be used to gain access to any other room. The cleaner needs to gain access to every room so rather than having a whole bunch of spare keys, she will have a master key that can be used to open every door. This would work best in a small hotel or guest house. If the hotel was a particularly large building over several floors there could be a need for a sub master key. In this situation each lock again would operate with their own individual key. The cleaner responsible for cleaning rooms on a particular floor would have a sub master key to gain access to each room on that floor but this key would not work on any other floor. Finally a grand master key would open every room on every floor.