Any key that can open two or more locks is considered a master key. Many master keys use pin and tumbler locks.
For a master key to work, though, there must be something called master wafers inside the lock. When the master wafers line up, the key and lock can turn freely. When they don't, it can't.
A master key is one that works with multiple master wafer configurations.
Take a look at the animated gif below and to the right (you can click it to view a larger version): The lock cylinders you'll find at Aim Lock & Safe comprise the following elements:
What makes a master key unique is that the locks have a third pin. That's called the master wafer and it can only be unlocked by a master key.
Do you see the tiny green elements in the image to the right?
Those are master wafers.
What makes them different is that a master wafer is a single piece (unlike the
red/blue pins seen in this image which are split in the middle.
After master key cutting, the key is then put into a lock with master wafers. When that happens, two shear lines are created.
In this image, there are two master wafers. But locks can have as many master wafers as needed, or they can have simply one.
When lock rekeying into a master key system, two keys are created:
So, how can two different keys open the same lock?
If you remember‚ locks with master wafers inside create two shear lines when a key is inserted. One shear line is created above the pin without a master wafer and one is created with the pins below the master wafer.
That means there are two ways to open the lock. The change key can only open it one way (with the shear line above the pins) while the master key can open it both ways (shear lines above and below the pins).
And that's how a master key works. One master key for all your locks.