How does a master key work?

The super of our apartment building is always going into everyone's apartment when something needs to be fixed. There must be a hundred apartments, but he only carries one key around with him. How does he get into all those apartments?

Master key system one key that would work in all locks

All you need to know about Toyota Keys Mechanical and Smart keys

How Does a Master Key Work? - Aim Lock and Safe

All You Need to Know About Master Keys

How Does a Master Key Work? - Aim Lock and Safe

How does a master key system work? - Locksmiths of Bromley

Master Key Systems Explained

How to make a Master Key.flv

How does a master key system work?

How does a Pin Tumbler Lock work?

How Does A Master Key Work? - Guardian Safe & Lock LLC