How come Key A can open Lock A. Key B can open Lock B. But not lock A. And key C can open all the locks? Doesn’t that mean A and B should be able to open each other?
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ELI 5:
To open the lock you must rotate a gear to pull back the bolt.
The gear doesn’t want to rotate because there is a pin that has dropped down a chute and blocked the turn. Push it up out of the way, and the gear will turn.
To lift the pin, you insert a key to push it up and out of the way.
This is what most old style locks were. Simple, but effective for most uses. However this is a very easy to beat lock. Push the pin up far enough and the lock opens. No real challenge.
The solution is to make it so pushing up to far also locks the gear in place. So the locking pin is now 2 pins sitting on top of eachother. Push it up just far enough that the break point is on the border of the spinning section and the stationary section and the key/gear will turn. To far up and the bottom pin binds. Not far enough and the top binds.
This is most modern locks.
Master Keyed locks have 2 breaks. So 3 pins.
The first break is for the normal key, and is unique to each lock. (or perhaps a small set, like “storage closets”)
The second break is identical across all locks in the set, and is the master key. (this would be all doors in the building)
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