A master key system is a hierarchical security arrangement that allows different keys to open specific doors while a single "Master Key" can open all of them. This is common in office buildings, apartment complexes, and hotels where access control needs to be both restricted and convenient.

How the Hierarchy Works

In a master key system, keys are organized in a "tree" structure:

• Change Key (User Key): This is the lowest level. It opens only one specific lock (e.g., an individual office or apartment door).
• Master Key (MK): This key can open every lock within a specific group or department. For example, a manager might have a Master Key that opens all offices on their floor.
• Grand Master Key (GMK): This key sits above the Master Keys. It can open every lock in multiple departments or an entire building.

The Mechanics: How One Lock Opens for Two Different Keys

To understand how this works, you have to look at the Pin Tumbler mechanism inside the lock cylinder. In a standard lock, each chamber has two pins: a bottom pin (which touches the key) and a top pin (driver pin). The lock opens only when the "shear line" (the gap between the pins) aligns perfectly with the edge of the cylinder plug.

In a master keyed lock, a third pin called a Master Wafer (or Master Split) is added between the top and bottom pins.

• Two Shear Lines: By adding this wafer, you create two possible shear lines in one chamber.
• The Change Key aligns the pins at one shear line.
• The Master Key aligns the pins at the second shear line (above or below the wafer).

Because either key can create a clean break at the shear line, the lock will turn for both.