BrightKidz Library
Subjects
Pins jamming a lock, and the same pins lifted clear Two separated cutaway exhibits, each showing the inside of a lock as two open frames stacked with a narrow gap between them: the fixed outer part above and the turning part below. In the left exhibit three solid bars hang down from the upper frame and reach across the gap into the lower one, so the two frames are pinned together and nothing can turn. In the right exhibit each bar has been lifted and split into two, an upper piece and a lower piece, with the break in every one of them sitting exactly in the gap. A stepped blade lies along the bottom of the lower frame, and the height of each step is what holds its own bar at the right level. Nothing crosses the gap any more.

What Is Actually Inside a Lock

About 13 minutes

Take your front door key out and look at it edge on, along the bumpy side.

It is not a smooth shape. It is a set of little valleys cut down into the metal at different depths, one after another. Count them. On most house keys there are five, sometimes six.

Those are not decoration and they are not there to make the key grip. Each valley is a measurement, and there is one pin inside the lock waiting for each of them.

The gap that everything depends on

A lock is two parts that fit inside each other. There is a fixed outer shell, screwed into the door and going nowhere. Inside it sits a cylinder that can spin, and it is the spinning of that cylinder that pulls the bolt back.

The join between the two — the thin circle where the turning part meets the fixed part — is called the shear line. Everything about a lock is about that line.

Because if anything at all is lying across it, the cylinder cannot move. Push a single matchstick through from the outside into the inside and the two parts are pinned together. That is not a metaphor. It is exactly what a lock does to itself all day.

One pin stack, with the right key underneath it A close cutaway of a single stack inside a lock. At the top a coiled spring presses down on a short bar called the driver pin. Below that sits a second bar, the key pin, and directly under it lies the stepped edge of a key. A dashed line runs straight across the drawing marking the shear line, which is the join between the fixed outer part of the lock and the part that turns. The step in the key has raised the key pin by exactly the right amount, so the place where the two bars meet lands on the dashed line and nothing is left lying across it. spring driver pin key pin the key the shear line

Drilled down through both parts are five little wells, and each well holds a spring and two pins stacked one on the other. The upper pin is called the driver pin and every single one of them is the same length. The lower pin is the key pin, and these are all different lengths.

The springs push down. With nothing to hold them up, every stack drops, and every driver pin ends up hanging down across the shear line — five matchsticks, jamming the cylinder in five places at once.

What the right key does

One pin stack, with the right key underneath it A close cutaway of a single stack inside a lock. At the top a coiled spring presses down on a short bar called the driver pin. Below that sits a second bar, the key pin, and directly under it lies the stepped edge of a key. A dashed line runs straight across the drawing marking the shear line, which is the join between the fixed outer part of the lock and the part that turns. The step in the key has raised the key pin by exactly the right amount, so the place where the two bars meet lands on the dashed line and nothing is left lying across it. spring driver pin key pin the key the shear line
  1. No key. The springs push every stack down as far as it will go, and each driver pin is left sticking through the join. The turning part is pinned to the fixed part in five places.
  2. A wrong key. Each valley lifts its own stack by some amount, but the amounts do not match the pins. One stack is lifted too little, so its driver pin still hangs across the line. Another is lifted too far, so now the key pin is poking up across it instead. Either one is enough.
  3. The right key. Every valley is cut to exactly the depth its own pin needs. In each stack the join between the two pins comes to rest on the shear line — and because every valley is right, all five arrive there at the same moment.
  4. Nothing is lying across the line any more. The cylinder is loose. Turn the key and the whole thing turns with it, and the bolt comes back.

Look at what that means for the key. The lock never checks what your key looks like. It has no way to. All it can report is five heights, measured one after another along the blade, and whether each one happened to be right.

A key is a written number, in metal, read by touch.

One valley on a key is cut too shallow, so it lifts its pin higher than it should. What happens?

That is the part people find surprising. A pin can be wrong in two different directions, and both of them jam the lock just as solidly. Too low leaves the upper pin hanging down through the join; too high shoves the lower pin up through it. There is exactly one height that works, and it is different for every well.

Five pins, and each one has to be right at the same instant as the other four. That is where a lock gets its stubbornness — not from being strong, but from asking five questions at once.

Count them yourself

You can check every claim in this article from the key in your pocket, without opening anything.

Hold two keys from the same house side by side, edge up. The overall outline will be near enough identical — same length, same shoulder, same grooves down the flat. Now look only at the valleys. Different depths, in a different order. Those two facts are the two locks working: the outline gets both keys into their slots, and the depths decide which door opens.

Then count the valleys and you know something real about the lock across the room. Five valleys means five wells, five springs, and ten pins, all sitting in the dark holding a line.