What is a Master Key?
A master key is a key which opens each lock within that specific system. Many people share the misconception that a master-key is a key that can open all or many locks solely by its own design. A master key is created by the locksmith who fits the locks to a desired system, it is done with much purpose.
Your key will open your door. So will your landlord’s master key! But your key won’t open your neighbor’s door… but the master key will! In a set-up like an apartment building, each door is keyed differently so your neighbors can’t get into your apartment, however the landlord has a “master key” that will open every door in the building. This is convenient for the landlord, as now they only have one key to keep track of, rather than many.
A master key is created when a locksmith fits pin-tumble style locks to multiple keys allowing the master key to remain constant throughout the system. We fit multiple keys to multiple locks by adding in additional master-pins creating multiple shear-lines for the lock to turn at. The purpose of this is often to simply life for people who require access to all or multiple parts of a building when at the same time others need to have more restricted access to some areas of the same building. A manager may have a key that opens the front door, the offices, and the storage closets while at the same time a employee has a key which only opens the front door and his specific office. If you own a building you may consider have a master-key system tailored to your specific needs. Master-key systems are a great way to control access to different parts of a given building.
Often when I re-key homes for people I offer to master-key one exterior knob, a perfect candidate would be the side service door to the garage. The idea is that they could leave a contractor a key to access the home, but limit his access. If I master-keyed one door knob but left your deadbolt keyed only to your personal key you would be able to temporally give a person access by leaving the deadbolt unlocked, borrowing him the secondary key that only works in that one specific doorknob. In most cases I believe it is preferable to leave your deadbolt unlocked for a few hours rather than lend the key to your home to a unknown person. This allows you to truly give temporary access with the conventional locks you already own.