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

Your key is a series of numbers (called cuts). When you try to use that key, you test if the lock has the same numbers. You can keep some of those numbers the same for all doors in your building and only test for those on the front door and use the rest for individual apartments.