I moved into an apartment complex where the key is both unique to each apartment door, but every key can open the front door, how are both possible?
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I see a ton of answers here saying each door reads half the key. I could be ignorant here, but ive taken a lot of locks apart and I’ve never seen or heard of this kind of setup.
Making a single lock open to multiple keys is known as *cross-keying*. This is done using master pins/wafers/spacers
In a lock, there are key pins and driver pins. When you insert your key into the rotating part of the lock, called the *plug*, the key lifts the key pins to just the right height so that the boundary between the key pins and the driver pins on top of them all lines up at the top of the plug. With the pins out of position, the plug is prevented from rotating because it hits one or more pins. With them at this special height, the plug can rotate with the key pins inside the plug and the drivers outside.
Splitting either of these pins created a new height where everything lines up. This is done by using shorter pins and adding in a master pin (also called a master wafer, and similar names).
So in the simplest situation, your apartment key has exactly one key that opens it. Your front door has spacers in it so it also opens to that key as well. In order to keep down the number of spacers your front door needs, it’s common to have some shared heights between the keys that open individual apartments.
For example, your key might have the heights 12345 and your neighbor might have 15739. You both share the 1 so that the front door doesn’t need so many spacers that every key that fits in the lock also opens it.
It’s bad practice, but for a large enough building different apartments might share the same key. Or the front door might have one or two sets of pins taken out. If there’s no fifth pin, than any key that gets the other pins right will work, regardless of the fifth pin
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