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?

How exactly does a “master key” work on multiple different locks? If a lock is made and a key is made to fit specifically into it, how does a master key that's

Master Key Installation Expert Commercial Locksmith Services

What is a Master Key System? - Locksmith Dubai Blog

Keys to the City – BLDGBLOG

What is a Master Keyed System?

What is a Master Key?

What is a Master Key System and How Is It Useful? • Assured Security

How Master Key Systems Work, Lockout Locksmiths

What is a Master Key System - Rekey

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

What Is A Master Key System & How Does It Work?

What is a Master Keyed System?