LOCKSHow a pin tumbler lock works

The part that turns and the part that doesn't

Inside a standard deadbolt or door knob there's a plug, a small round core the key slides into, sitting inside a housing. The plug turns and the housing doesn't. The two together are what locksmiths call the cylinder. The lock opens when the plug is free to rotate and move the bolt or latch. The pins and springs are in there to stop it from turning.

Pins, springs and the shear line

A row of small holes is drilled down through the housing and into the plug, usually five or six of them on a house lock. Each hole holds two stacked pins and a spring pushing down on them. The bottom one is the key pin. The top one is the driver pin.

With no key in the lock, the springs push both pins down far enough that the driver pins cross the gap between the plug and the housing. They act like a row of tiny bolts pinning the part that turns to the part that doesn't. Nothing rotates.

That gap has a name: the shear line. It's the whole game.

What the key does

The cuts along the top of the key lift each pin stack by its own amount. They're cut to the exact depths that bring the top of every key pin level with the shear line. The driver pins sit up in the housing, the key pins sit down in the plug, and nothing crosses the gap. The plug turns and the bolt moves.

A wrong key lifts some pins too far or not far enough. Lift one too little and its driver pin is still across the shear line. Lift one too far and the key pin itself is across it. Either way at least one pin is in the way, and the plug stops dead.

Why this matters for rekeying

On a lock like this, rekeying means swapping the key pins for a set of different lengths, so a different key lines up at the shear line. (Some newer house locks are built differently and get rekeyed with a small tool instead of new pins.) The old key now lifts the pins to the wrong heights and stops working. Same lock, same cylinder, different pins.

That's why rekeying usually costs less than replacing a lock, and why it's the usual fix when you've lost a key or moved into a place where you don't know who has copies. Rekey or replace? How to decide.

When a lock sticks, it often isn't the lock

A key that feels gritty or won't turn smoothly is usually a worn key, a worn cylinder, or dirt. A lock that works fine with the door open but binds when the door is closed is almost always a door problem: the latch or bolt isn't lining up with the strike plate in the frame. Here's how to find the cause.

Not every lock is a pin tumbler

Many car door and ignition locks use thin flat wafers instead of pins. A mechanical safe dial lines up a stack of wheels. Higher-security locks add extra parts, like a second set of pins or a sidebar, so that lifting the pins to the shear line isn't enough on its own. The idea of lining something up so a part is free to move shows up in all of them.