Master Keying and Key Control Systems
Hierarchy and mechanics
Master keying is both a mathematical system and an administrative program. A change key operates one lock; a master key operates a group; a grand master operates several masters; a great grand master operates all. Each added level increases the number of keys that open a given lock and reduces effective security. Know bitting (cut depths on a key), the shear line, pin stacks (bottom pin, master pin, top pin and spring), and MACS (maximum adjacent cut specification), which limits how much adjacent cuts may differ so keys enter and operate smoothly. Restricted keyways and patented blanks support key control, but they do not replace policy, records, and audits.
Traps
The classic trap is treating a Do Not Duplicate stamp as legal protection; it is a request, not a control. Another is assuming a restricted keyway prevents all duplication, when blank control and patent protection may be absent. Questions also confuse keyed-alike (one key for a group of locks) with master keying (hierarchical access). Over-master keying produces ghost keys that open locks they should not. Construction keying must be removed and the cylinder re-pinned at turnover.
Worked example
A change key cut is 5 and the master cut for that chamber is 7. Pin the chamber with a bottom pin of 5 and a master pin of 2, so the change key shears at 5 and the master key lifts the extra 2 to shear at 7. Rule of thumb: bottom pin equals change key cut; master pin equals master cut minus change cut. Audit issued keys against returned keys and re-pin at turnover.