Master Key Systems
A master key is a set of several locks. Usually, there is nothing special about the key itself, but rather the locks into which it will fit. These locks also have keys that are specific to each one (the change key) and cannot operate any of the others in the set. Locks that have master keys have a second set of the mechanism used to operate them that is identical to all of the others in the set of locks. For example, master keyed pin tumbler locks will have two shear points at each pin position, one for the change key and one for the master key. A far more secure (and more expensive) system has two cylinders in each lock, one for the change key and one for the master key.
Larger organizations, with more complex "grandmaster key" systems, may have several masterkey systems where the top level grandmaster key works in all of the locks in the system.