A key which operates the entire master keyed locks in a group, each lock is usually
operated by its own key.
Can be combine as a group of locks so that each is operated by its own change key as
well as by a master key for the entire group of locks.
There are many benefits of a master keyed system, but the overriding one is that
access can be controlled across a building allowing appropriate people access to
defined parts of the building. Secondary advantages mean that individuals can have
access using only a minimum number of keys.