Masterkeying
Replace this key

A key is stolen. Re-pin one lock, not thirty.

Unit 10 in a thirty-unit block loses its keys. The usual answer is to re-key the whole system and cut everyone a new key. The older answer — and the one Masterkeying works out for you — is to re-pin only the cylinders that key operated onto a ghost bitting the progression never issued. Every other tenant keeps the key in their pocket.

Try it on a unit block Open the designer
How it works

Four steps, all proved before you see them

Pick the key

Any key on the schedule except the top key. The plan covers exactly the cylinders that key operated and no others.

Find the ghost

Every bitting the progression could have produced near the old one is tried against every cylinder — nobody holds it, nothing it was never meant to open opens, no stack goes over three master pins. The cheapest in pin changes wins.

Re-pin the cylinders

Per cylinder: the stack before, the stack after, and the change in plain words — remove a 2 master pin; add a 4 master pin. Most cylinders are one pin out, one in.

Prove the lock-out

The old cuts are simulated against every cylinder as it will be pinned. They must open nothing — or the plan names the one kind of door where they still turn, before you apply anything.

What you get

  • The new cuts for the replaced key, and the old cuts on a retired list that is never cut again — the list travels in the share link.
  • Every affected cylinder with the pins to change, chamber by chamber. Untouched cylinders are counted, not listed; they stay as they are.
  • The easy change key per cylinder: the cut that lifts the stack so the pins to change sit at the top of the plug when it is pulled. It is almost always the stolen key itself, cut from code — insert it, pull the plug, and the wafers you need are on top.
  • The proof. A retired-bittings matrix on the schedule: each old key against each cylinder, blank where it is locked out.

The honest limits

Maison doors. A lobby or bin-room cylinder that every tenant’s key opens holds every tenant’s cuts stacked together. If each of the stolen key’s cuts is also held by a key that must keep working, no ghost can lock it out of that door without re-cutting other keys. The plan says exactly that, names the door, and names the cheapest set of keys that would need re-cutting to free a chamber — or tells you to move that door onto its own common key, or to accept it. It never calls that a clean success.

Master keys. A master’s cuts live inside every key under it, so its cylinders go on accepting it whatever ghost you choose. A lost master means re-cutting the keys under it. The designer refuses and shows which key holds each cut.

The register must match the brass. The maths assumes each cylinder is pinned the way the schedule says. Before you trust a plan on site, pull one affected plug and check.

Ghost keys are a feature of every mastered system, not a flaw in this one. The rule a designer enforces is “no issued key operates an unassigned door” — never “no spare bittings exist”. The designer’s uncut keys that also fit column counts them per cylinder; a replacement is chosen from exactly that pool.

Where it runs

Today, in the free designer on this site, on any system you generate there — nothing saved, the design and its retired list live in the link. The app’s saved projects, with their cutting register, are next in line for it.

Questions

What is a ghost key?
A bitting the progression could produce but nobody has been issued. Every mastered cylinder accepts some. Replacing a lost key means re-pinning that key’s cylinders so they accept a ghost instead of the lost cuts.
Do I have to re-key the whole system when a key is stolen?
Usually not. Only the cylinders the stolen key operated change, and on most of them it is one master pin out and one in. Every other key keeps working. The exception is a shared maison cylinder where every cut of the stolen key is also held by a key that must keep working — the plan tells you when that is the case.
Can a lost master key be replaced the same way?
No. A master’s cuts are inside every key under it, so its cylinders still accept it. Replacing a lost master means re-cutting the keys under it; the designer refuses honestly and names the holders of each cut.
What is an easy change key?
A cut that lifts each affected stack so the master pins to change sit at the top of the plug when it is pulled. Almost always the stolen key itself, cut from code; the plan gives a service cut when the stolen cut won’t do.
Why can a replaced key be replaced again?
Each replacement retires the old cuts. The next ghost is chosen from what is left, and no retired bitting is ever re-issued or allowed to gain a door through a later change. The retired list is part of the share link.