In the locksmithing world, the term "Master Key" can be confusing. At a first guess, one may think that this key allows access to any lock on any door. In reality, a master key is a key that can open multiple doors that use the same keying system. These keys can be useful in apartment scenarios, where each tenant has their own key, that opens only their front door. The landlord has a master key, which allows access to every apartment, without having to carry one key for each apartment.
Many master key systems work by adding a third pin to one or more of the slots. Each pin set normally has just two pins in it. When the correct key is inserted, the pins line up and let the cylinder turn. When a third pin is added, there are two key depths that can line up the pin properly. One can be used for the tenant, and the second can be used for the landlord's master key. Using a third pin in one or more of the slots adds to the combinations that can open the door. By designing a system with care you can have keys that only open a single door, and others that open a certain set, or all of them.
This is all done by working out how many keys you need, and what you want them to do. Tracing where employees need to go and giving them a key that opens any doors they need access to, while keeping other doors secure. Those people that need access to the whole building, like the site manager, maintenance etc. can have a more universal key.
It is very important to plan how the system is going to work. This means asking the customer how they would like the system to work. Adding in keys for future expansion is also useful. If the need arises, other keys can always be added. The hierarchy of a master keyed system generally looks like the one found below, although the terminology may vary. Above the Grand Master key is the Great Grand Master key. With each subsequent layer, another Great is added.