A pin tumbler cylinder holds a row of spring-loaded pin stacks in the housing above the plug. Each stack is a bottom pin that touches the key and a driver pin above it, pushed down by a spring. With no key in, the drivers straddle the gap between the plug and the housing, and that gap is the shear line. The plug cannot rotate while any driver sits across it.

Each cut on the key blade is milled to one of a small set of depths. When the key slides in, the cuts raise each bottom pin by the amount that puts the join between bottom pin and driver level with the shear line. Once every join lines up, the plug is free, and the key turns the cam or bolt behind it.

The depth of a cut is the information the lock reads. The pins respond only to how high the blade pushes them, so a change of a fraction of a millimetre at one cut can leave one pin stack a little short of the line.

House keys are cut from brass or a similar soft alloy. Brass cuts cleanly and is gentle on the pins, which is why it is chosen. The trade-off is that it is the softer of the two metals in contact, so it takes the wear first.

The contact is concentrated. Every pin rests on the peak of a cut, and as the key is pushed in and pulled out, those peaks drag across the bottom pins. Over thousands of uses, the sharp shoulders of each cut round off and the valleys between them lose their definition.

A rounded peak sits lower than a crisp one, so the pin it should raise to the shear line rides slightly short. One pin that is a touch low can be enough to hold the plug, which is why a key can work for years and then need coaxing on a single door while its spare opens it easily.

When a pin stack is short, the driver pin still sits partially across the shear line, holding the plug. Wriggling the key, lifting it slightly as you turn, or pulling it out a hair while twisting changes the angle the blade presents to the pins and raises the short stack the last fraction. The plug turns, the door opens, and you learn that jiggling works.

That learning is what does the harm. A pin pressed against the shear line under a turning load is dragged across the edge of its chamber in the plug and the edge of the housing. Repeated over months, that scraping can wear the pin tips and round off the edge of the chamber, and the cylinder starts to bind at positions the original key once reached cleanly.

The turning force goes into the key as well. A brass blade already thinned at its cuts has less metal across its weakest section, and added force from a stiff lock or a hand levering on the bow can shear the blade off in the plug.

Wear shows up in the hand long before it shows on the blade. A key that turns only once it has been lifted, eased back, or rocked is the plainest sign. Others come from the same short lift: a turn that now needs wrist effort where two fingers once did, a key that drags on its way in or out, and a lock that works first time one day and on the third go the next. The telling check is the spare that has lived in a drawer: if it turns the same lock smoothly, the daily key has worn.

The spare test separates a worn key from a worn lock. A smooth spare points at the original key. If the spare is rough too, the lock has taken some wear, or the door and keeper are loading the bolt. A door that has dropped on its hinges can make any key feel stiff, so try the key with the door held open before blaming the cylinder.

Visible clues exist as well. Hold the key beside a newer one with the blades side by side: the worn key shows shallower, rounder valleys and a polished shine along the contact edges.

A copy takes its shape from the key it is traced from. If the original has rounded peaks, a duplicating machine traces them and cuts them into the new blank, so the wear is copied along with the pattern. The copy opens the lock with the same reluctance, and it begins life already worn.

Copying a copy repeats the loss. Each generation carries the tracing error of the last, and the small differences can stack up to a depth the pins notice.

A better source is the lock's own specification. A key can be cut to code from a stamped number or from the depths read at the pins, which gives each cut its original depth with no wear. The next choice is the least-worn key in the household, and a locksmith can decide which key that is by checking each blade against the lock.

Once the pins and chamber edges are scored, a new key cut to the original depths may not cure the roughness because the lock now carries its own wear. That is why timing matters. A fresh key cut while the lock is still sound restores the original lift and prevents jiggling before the pins and plug are damaged. A lock that is already damaged may need its pins renewed, its cylinder rekeyed, or its cylinder replaced.

A fresh key is a short job: cut, test and finished in one visit. Repairing worn pins and plug means taking the cylinder out of the door, stripping it and rebuilding or replacing it, a longer job than cutting a key. The key is the replaceable part, and the lock is the part that has to be protected from it.

Acting at the first sign of jiggling keeps the repair at the key. Waiting until the key stops working at all risks a snapped blade in a damaged plug, which turns a key job into a lock job.

Wear is gradual and quiet, and a key wears at the rate it is used. The key on the daily ring is the one to watch, and a stiff turn is the time to act.