Many times for commercial buildings, the owner or landlord does not want to have a key for every unit. Instead they prefer having a master key that can provide them access to every room. In addition, there are grandmaster and great-grandmaster key options as well.
Master Key System
A master key set up is a two level system. The high level keys are called the master keys and the lower level keys are called the change keys. The change keys will only operate one lock, while the master key is able to open any of the locks.
Grand Master Key System
Multiple two level systems can be joined with one key that controls them all creating a three level system. Each section has a different master key that works for all change key locks under it but the grand master key is able to open any of the change locks under any of the master keys.
Great Grand Master Key System
Multiple three level systems can be joined with one key that controls them all creating a four level system. Two sections can be opened by one grand master key, but either of the sections including all the locks under it can be opened by the great grand master key.