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

You could have the first three pins the same on every key, and the front lock only has those three pins and the rest of the lock is just empty, turning freely once the three pins are aligned correctly (from inserting any of the keys).

Then the individual apartments have seven or more pins, with the last four+ pins uniquely aligned at each apartment.

So only one key can turn the last four+ pins per apartment, but all the keys can turn the front 3 pins at the front gate.

(This is just one of many ways to accomplish the same thing, and the numbers I used here are totally made up guesses.)