Keying, Master Keying, and Key Control
Core concepts
Keying begins with the key bitting: the cuts on a key correspond to pin lengths. Know the difference between a change key (operates one lock), a master key (operates a group), a grand master, and a sub-master. Understand the master-keying hierarchy and the two-level pin stack (bottom pin + master pin) that allows multiple keys to align at the shear line.
Key control covers the administrative side: restricted keyways, patented keys, key registers, issue/return logs, and rekeying on turnover. Understand that master keying reduces security by increasing the number of keys that open a given lock and by introducing master pins that create pick and bump vulnerabilities.
Traps in questions
- Confusing change key with master key.
- Forgetting that adding master pins increases the number of shear-line combinations.
- Assuming a restricted keyway is unpickable - restriction limits duplication, not picking.
- Overlooking the need to document key issuance.
Rule of thumb
Total pin stack = bottom pin + master pin(s) + driver pin, and the stack must equal the chamber depth. When building a master-key system, verify each key's bitting against every lock it must open before cutting metal.
Worked example
A building needs a master key for all offices and individual change keys. Assign a two-level system: each office lock gets a unique bottom pin, a common master pin, and a driver. The master key's bitting must align every lock at the shear line; the change key must align only its own lock.