Change key

A change key — also called a leaf key or operating key — is the lowest level of key in a master key system. It opens exactly one cylinder, or a small set of cylinders that have been keyed alike. It’s the key the end-user actually carries.

In a typical office building, the change keys go to individual office doors, each opening just one room. The master keys go to floor managers (one per floor zone), and the TMK goes to the building owner.

How change keys relate to the TMK

Every change key in a master keyed system is bit-related to the TMK. In each chamber where the cylinder has a master pin, the change-key cut and the TMK cut differ by the master pin’s height. In chambers without a master pin (some allocation methods like RCM use these intentionally), the change key and TMK share a cut depth.

This is why losing a change key can affect TMK security: a phantom analysis on the system might reveal that the lost change key’s bitting, combined with master pins from another cylinder, opens cylinders it was never authorised for. If the phantom coverage wasn’t tight at design time, a routine change-key replacement turns into a rekey.

Change-key keying patterns

• Keyed differently (KD) — every change key has a different bitting. The default for commercial MK.
• Keyed alike (KA) — multiple cylinders in a group share one bitting. Common for office suites where the occupant wants one key for several doors.
• Selective master keyed (SMK) — change key opens one or several specific cylinders defined at design time, not via a hierarchy.

Related

→ TMK — the master at the top of the hierarchy → Grand master key — when a TMK isn’t enough → Master pin — the mechanism connecting a change key to its master