How Master Key Systems Works
Did you ever wonder how the maintenance guy can open every door in your building, yet all he carries with him is one key? The key he carries is a master key, and while all other keys may only open one of the doors in the building, his key is not limited at all. There is nothing special about the actual key in a master key system. The real explanation lies actually within the locks themselves.
Why Master Key Systems are Needed
To make things easier to understand, we’ll just use an example of the most basic master key system. Imagine you had two doors, each door had its own individual key which unlocked it. In addition you now wanted one key for yourself which would unlock both doors. This is where master keying would used. A locksmith would rearrange the pins inside the cylinders of your locks in such a way that would allow your previous individual keys to still work, but at the same time you would have one key that opened them both.
At a larger scale, imagine a building where you had over a thousand doors, a hotel for instance. Now you would like to give each guest a key that would only work his room, you would want the maid to have access to all the rooms but limited to certain floors in the building. And on top of all that, the hotel manager would have one key that would unlock everything. This would be a three level master system, since now you have multiple levels of authorized entries.
Of course, this is an overly simplified explanation of how Master Key systems work, but it should provide a good starting point.