A pin tumbler cylinder has a plug that turns inside a fixed body. Each chamber holds a stack: a bottom pin resting on the key, a driver pin above it, and a spring pushing both down. With no key in, the driver pins sit across the split line between the plug and the body, and the plug cannot turn. The right key lifts every stack until the gap between bottom pin and driver pin lands on that split line. A wrong key leaves at least one pin straddling it.

A construction keyed cylinder adds one loose part to one chamber, a small ball sitting in the stack. At that chamber, the builder's key is cut deeper than the owner's key by the height of the ball. With the builder's key in, the ball stays inside the plug, the break in the stack lands on the split line, and the cylinder turns.

The owner's key, cut shallower at that one position, lifts the ball clear of the plug and up into the body. When the plug rotates, the spring presses the ball down into a small trap hole drilled into the plug, and there it stays. The stack in that chamber is now one ball shorter. The deeper cut on the builder's key no longer raises it far enough, so a driver pin drops across the split line and the plug will not move. The owner's key, cut for the shorter stack, continues to work as normal.

Two things follow from that mechanism. The change is permanent: no key can lift the ball back out of its trap. And it happens one cylinder at a time, only in a cylinder where the owner's key has been turned. A cylinder that never sees the owner's key stays open to the builder's key indefinitely.

The strongest case is a renovation that adds or replaces several external doors while the house stands empty: a rear extension with new sliding and hinged doors, a second storey with its own entry, or a whole-house refit where every cylinder is being replaced anyway. Plumbers, electricians, plasterers, tilers and painters each need to get in on their own days, and handing out copies of a key that will one day open the finished home is the risk construction keying removes.

The case weakens when the owners keep living in the house through the works. Doors that are not changing stay on the owners' existing keys, and someone at home can let trades in. Construction keying then covers only the new cylinders, and it may cover very few of them.

A one-door job sits in between: a temporary cylinder swapped out at the end does the same work, and on a single door the swap is small. Construction keying pays off where the new cylinders are permanent, and several trades need them over months.

Renovations across several dwellings at once, such as an older house split into two, gain a further step: each unit's own keys close off the builder's key as that unit is handed over, so a finished unit can be lived in while work carries on next door.

Old locks that stay on the house sit outside the system entirely. The builder's key will not fit the original mortice lock or rim latch being kept, so if trades need to use that door, someone will lend them its key. Any lock whose key went out to the site that way is worth rekeying when the job ends.

The sequence that keeps the system tight runs from the hardware order to the final walk-through:

The mechanism only fires where the owner's key is turned, so the gaps are the cylinders nobody turns. A gate used once a season, a sliding door that stays shut through winter, or a storeroom door on the far side of the house can sit on the builder's key for years if handover skips it. The list from step one is what catches them.

The second gap comes from changes made during the job. If a door is damaged and a replacement cylinder is fitted by someone outside the plan, that cylinder may be keyed only to the builder's key with no owner position at all. It will keep obeying the builder's key for good, because there is no ball for the owner's key to move. Every cylinder added after the original order belongs on the list, with a note of who supplied it and how it was keyed.

The third gap is an owner's key used early. An owner checking a new front door during a weekend visit can end the builder's access to that door months ahead of schedule. Nothing is broken, but the trades now need the owner to open that door for them until handover.

A locksmith belongs at the start, when the keying plan is drawn up, and the cylinders are pinned. The choices that matter (which doors get a construction position, how many builder's keys exist, whether the new cylinders are keyed alike or sit on a master key system for a later stage) are all fixed at pinning.

Bring the locksmith back if any of these turn up at handover:

- A builder's key still turns after the owner's key has been used in that cylinder.

- The owner's key binds or will not turn in a new cylinder. Forcing it risks bending or snapping the key inside the plug.

- An old lock that stayed on the house had its key lent to the site.

- A cylinder fitted mid-job has no record of how it was keyed.

Construction keying asks very little of anyone on site. The cylinders do the work, provided the owner's keys stay out of them until the end, and every cylinder gets its turn on the last day.