A key opens a lock because the depths of its cuts match the pin heights inside the cylinder. That match is the whole of its power, and it is useless to a finder who cannot place the lock. So the risk from lost keys rests on two questions: what travelled with them, and where they went missing.

Inside a standard cylinder sit pairs of pins in a row of small chambers: a key pin at the bottom, a driver pin above it, and a spring on top. With no key in, the springs push the driver pins down across the gap between the plug (the part that turns) and the outer shell. That gap is the shear line, and while pins straddle it, the plug cannot rotate. The right key lifts every pair so that the split between the key pin and the driver pin lands exactly on the shear line, and the plug turns.

To rekey, a locksmith removes the cylinder from the door, slides the plug out with a follower tool so the driver pins and springs stay in their chambers, and swaps the key pins for lengths that suit the new key. The lost key now leaves the pins at the wrong heights and will not turn the plug. The lock body, bolt, handle, and strike plate stay where they are. The last step is trying an old key in each rekeyed cylinder to confirm it no longer turns.

Not every lock on a property works the same way, and the mix matters when deciding how quickly this can be done.

The table below sets out the common combinations and what each one tells a finder. The point is to decide how fast to act, since a plain key lost far from home and a key ring stolen with your licence do not call for the same response.

Look at the whole ring, too. Keys for a side gate padlock, a shed, or the internal door from the carport each open another way onto the property, and they belong on the list of locks to deal with.

A lost key ring with your address on it is a short job list, and most of it can be done before a locksmith reaches you.

For the problem of lost keys, it is. Changing a lock means matching new hardware to the door: the right backset, the same timber cutout, a compatible strike, and sometimes filling or recutting the door where the old and new shapes differ. Rekeying skips all of that, because the hardware never leaves the door except for the cylinder. A standard cylinder takes about 15 to 30 minutes, and a full set of doors can be done in one visit with one new key for every lock.

Replacement comes into it when the lock itself has a problem. A cylinder that is broken, or worn badly enough that the pins and plug no longer hold a clean shear line, is not worth rekeying, and a lever mortice lock with no practical way to change its key may be swapped for a new one. The lost keys are the reason for the visit; the condition of each lock decides which job it gets.

A master key system adds a layer that the lost key affects. The cylinder in your unit door is pinned to accept your key and at least one master key that opens a group of doors. Your key may also open the common entry, a bin room, or a car park door. Rekeying your unit cylinder has to keep the master working, which means the locksmith needs the system's keying details, and those are held by whoever manages the building.

Some buildings use a restricted key profile, where blanks are controlled, and copies are cut only with authorisation. A lost key from such a system cannot be copied at a hardware counter, which lowers the risk of extra copies circulating. It does not stop the original key from working on your door, so the rekey still matters when your address is missing from it.

A lost key ring rarely puts anyone in danger by itself. What turns a nuisance into a real exposure is the scrap of paper, card, or tag that travels with it, and the fix is the same whichever way it went: count the doors, lock down what you can, and have every cylinder the lost key fits changed to a new key.