Free master key system designer

How to use it

• Keys. Optional. Create up to eight keys that open more than one door — a master per floor, a room key, a cleaner's key. The top key is added for you.
• Doors. Name each door (up to 16) and tick every key that opens it. Own key gives the door a change key of its own. Maison means every key in the system opens it. Nothing ticked means top key only. Several doors ticked the same way are keyed alike.
• Hierarchy. You don't pick levels — the designer reads them from your ticks. If every door key A opens is also opened by key B, A sits under B. Keys that share some doors but not others are cross-keyed. Up to three levels sit under the top key, which becomes a master, grand master or great grand master to suit.
• 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. No key on the schedule opens a door it isn't ticked on. On cross-keyed and maison doors the cylinder accepts every mix of its keys' cuts, so the designer checks each key against those mixes as it places it.
• Three master pins per chamber, tops. A chamber only holds so many wafers. Busy doors have their differences spread across chambers to stay inside it.
• 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 — and the proof matrix checks every key against every door 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. The last column counts the uncut keys that also fit — bittings nobody holds that the cylinder would still turn for. 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

Sixteen doors, eight shared keys (plus an own key per door), three levels under the top key, three master pins per chamber, six chambers at depths 0–9 — and no saving, no cutting register, no export. Those live in the Masterkeying app — projects that persist and can be revised later, bigger systems, 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 key. Fine and common: tick the same key on each door.
• Cross-keyed — a door that two or more keys open without either sitting under the other (a shared door between two rooms, say). Normal, but the cylinder carries more master pins.
• 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".