Free master key system designer

How to use it

• Doors. Name each door (up to 12 on the free page). Each gets its own change key.
• Sub-masters. Optional. Add up to three — one per floor, wing or tenancy — then assign doors to them. With sub-masters the top key becomes a grand master; without, it's a plain master.
• Top key. Leave it blank for a random even-odd start, or type six depths (0–9). Any legal key can seed a system.
• Generate. The engine plans the progression, then proves it. Read the schedule, the pin stacks and the proof matrix below.

What it enforces

• No cross-access. A change key never opens a door it isn't assigned to; a sub-master never opens another group's doors.
• Unique bittings. No two keys on the schedule are ever the same brass.
• MACS 7. Adjacent cuts never differ by more than 7 — on every key, and across every pinned cylinder.
• Master pins 2–8, even only. Progression cuts sit on an even-offset lattice from the top key, so every master pin is a 2, 4, 6 or 8 — the pins a locksmith actually carries. Never a 1, never a 9.
• Minimum-chamber mastering. A chamber is only mastered where keys actually differ; the rest stay keyed straight, which keeps the locks harder to pick.
• Level-partitioned progression. Each level of the hierarchy varies from its parent only in its own block of chambers, so phantom openings are impossible by construction — and the proof matrix checks anyway.
• Honest failure. If no valid system exists for your top key and door schedule, it names the blocking level or door instead of showing a broken chart.

How to read the result

Key bitting schedule — every key with its six cuts. This is what gets cut. Pin stacks per door — for each cylinder, the key pin and any master pins in each chamber; a chamber with a single number is keyed straight. Progression proof — every key simulated against every door at the shear line. OPEN where it should open, blank where it should be blocked, and a red cell if anything is wrong (you shouldn't ever see one).

Limits of the free page

Twelve doors, three sub-masters, one change key per door, six chambers at depths 0–9, and no saving. No keyed-alike groups, no maison doors, no cutting register, no export. Those live in the Masterkeying app — projects that persist, unlimited doors, a cutting register that tracks stamp numbers, and a free account that lets you save a new system every month.

Glossary

• Bitting — the sequence of cut depths on a key, one per chamber.
• Change key (CK) — the key that opens one door (or one keyed-alike group) only.
• Master key (MK) / grand master (GMK) — a key that opens every door in a group / every group in the system. Very large sites add a great grand master (GGMK) above that.
• Sub-master — the everyday name for a master key over one part of a bigger system: a floor, a wing, a tenancy.
• Chamber — one pin position in the cylinder. Six here; five and seven exist.
• Key pin / master pin — the bottom pin that sits on the key, and the extra wafer above it that creates a second shear point so a second key can operate the chamber.
• Shear line — the gap between plug and housing. When every chamber's split sits on it, the plug turns.
• Progression — the pattern of alternate cuts that gives each key its own distinct place in the hierarchy.
• MACS — maximum adjacent cut specification: how far neighbouring cuts may differ and still leave a key that cuts and turns. Seven here; it varies by manufacturer.
• Rotating constant / level partitioning — giving each level of the hierarchy its own chambers to vary in, holding the parent's cuts everywhere else.
• Keyed alike — several doors on the same change key. Fine and common; the free page keeps one key per door.
• Maison door — a shared door every tenant's change key opens (a lobby, a bin room). Convenient, heavily mastered, worth thinking twice about.
• Ghost keys — positions on the schedule deliberately left uncut for future doors or a replacement master. A feature, not a defect: the rule is "no issued key operates an unassigned door", never "no spare keys exist".