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

Imagine the key has eight teeth.

The first four are different for each key, and unlock your individual doors.

The last four are the same for each key, and open the main door.

Your exact arrangement will vary, but the concept will be the same.