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

Some keys/locks have TWO rows of teeth. In your example, every key is individual to its unit, but they are all master keyed to the front entry.