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This definition is very simple and it is both correct and incorrect.
If your business has 15 interior doors and all of the locks are keyed alike then your key will open all of those locks.
Some people will call this key a "Master Key".
This is not correct in terms of the lock and security business.
A better definition would be that a master key will open all locks that are under it and is on the top.
Let's look at those same 15 doors, this time we will break them down in three groups with 5 locks in each group.
In this scenario group 1 will operate with key 1, group 2 with key 2 and group 3 with key 3.
Each group key will only open the locks in that group.
It will not be able to open any other lock in any other group.
The master key will be able to open all locks in all groups.
This is a very basic System, they can get very complicated from here.
Let's take a look at what is called a Grand Master System.
In this system you would have a second group of 15 locks for a total of 30 locks or doors.
The second group would be set up exactly like the first described above, with its own master key.
The master key for each group will only open the locks in its own group, however the Grand Master Key will be able to operate all 30 locks in both groups.
From the Grand Master System the next step up is the Great Grand Master System, this system will operate the same as the previous systems.
These systems will get very complicated to the point where special computer programs designed to develop master systems will have to be used.
These computer programs have algorithms that will prevent cross keying, this is where a key in one group operate a lock in another.
They also have the capability to control the keys and assign the correct lock to the correct door in a system.
It is also important to understand that any master system will lower the security of any lock.
This is due to the fact that you will have more than one key able to operate a lock.
So you will give up some security for convenience.
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