COMMERCIALHow a master key system works

The extra pin

In a standard pin tumbler lock, one key lines every pin stack up at the shear line. In a master keyed pin tumbler lock, one or more of the stacks get an extra short pin called a master pin, set between the key pin and the driver pin. That gives the stack two heights that can line up with the shear line: one for the lock's own key and one for the master key.

Either key lifts the pins to a position where nothing crosses the shear line, so either key turns the lock. The shear line, explained.

Levels of keys

• The change key opens one lock, or a small set of locks that are keyed alike.
• The master key opens a group of locks, each of which also has its own change key.
• A grand master key opens several of those groups.
• Larger systems can add another level above that, called a great grand master.

Keyed alike is not master keyed

Keyed alike means every lock uses the same key, with no levels. A master key system lets each lock, or each small set of locks, keep its own key as well, so you can give a tenant or a department a key that opens only their doors while one master key opens them all.

The trade-offs

More ways to line the pins up also means more keys that happen to work. A master keyed lock will accept more keys than the same lock keyed on its own. The designer has to plan the key combinations so one change key doesn't open its neighbor's lock by accident, and that gets harder as the system grows.

Systems also get messy over time. Keys are lost, locks are added, and records go missing. When nobody knows what the original plan was, the system has to be figured out one lock at a time.

Why the records matter

Two records hold a system together. The bitting list is the record of the cuts on every key in the system. The keying schedule is the record of which key goes with which door. Without them, adding a lock or replacing a lost key means working the system out from the locks and keys on hand, and nobody can be sure which combinations are already in use.