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?

Question

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?

In: 986

Answer 1

To expand on the answers already here, it’s likely some combination of master wafers and shared key cutting…
A common lock has a bunch of pins with springs, when these pins are pushed up in the wrong way they don’t let the lock turn, like that board game rush hour where you’ve got to clear a path for one object to move through.
A key is shaped so that it pushes all the pins correctly out of the way to let the lock spin, and also helps you spin the lock.
A master keyed system allows one lock to take multiple keys by having more than one cut in some of the pins. Normally, as the name suggests, you’d have a bunch of locks that have different keys but one special key that can open all of them… there’s technically not much stopping you from doing it the other way around, but it does make the front door lock much less secure.