What is a master-key system?

I Found this word while reading an article about Locksmith.

It is a locking system where a number (even hundreds) of locks can have individual keys that only fit one lock and a "master" key that fits all of them. Imagine a large apartment complex where each tenant has a key that only fits their apartment and the maintenance people have a "master" key that can be used for emergencies and repairs to unlock any of the doors..

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Master key system is a mechanical access system whereby all doors open by a single key. This key operates a set of several locks. These keys work with different types of lock like door locks, cabinet locks, padlocks, etc. as long as the cylinder inside them is of the same model. Master key systems increase control and convenience, it has multiple levels of access. https://trilocklocksmith.com/