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

Each key is a code.

The number to the front door is 123xxx. The x numbers don’t get read.

The code to your apartment is 123957.

Your neighbor’s code is 123744.

The other neighbor is 123676.

With this example you could just tell everyone to put in their apartment code at the main door and they would never know the difference. Once they have pressed 123, the lobby door opens and the other three digits are ignored.