Philadelphia master key system

https://getsamedaylockrepairden.solsticebrief.com/posts/why-same-day-lock-repair-matters-for-philadelphia-homeowners

The Philadelphia master key system is a classic example of a pin tumbler lock design that uses a set of master keys to access multiple locks with different individual keys. It allows a single master key to open all locks in a system while each lock has its own unique key. This system is widely used in commercial and residential buildings for convenience and security.