Saramsha

No. 013  ·  29 September 2026

Five Pins and a Line

The lock on your front door is five little stacks of two pins each, pressed down by springs. The plug can only turn when every join sits on one line — and that is the whole of what a key does. Drag a pin upward to lift it.

Apply turning pressure, then lift a pin and feel which one resists.

About this piece

The only thing a key does is lift

A key looks like a code, and people talk about it that way — the right shape, the right pattern, the secret. It is nothing of the kind. Inside the lock there is one line, called the shear line, where a turning cylinder called the plug meets the fixed body around it, the shell. Five holes are drilled straight through both at once, and each holds two pins: a short brass key pin in the plug, a longer steel driver pin above it in the shell, with a spring pressing the pair down. At rest every driver pin hangs across the shear line, bolting the plug to the shell. The key's single job is to push each stack up by exactly the right amount so that the join between the two pins lands on the line. Five joins, one line, and the plug is free.

That is why the notches on a key are different depths: each pin is a different length and needs a different lift. A cut that is too deep leaves its pin low, and the driver pin still crosses the line. A cut that is too shallow pushes the key pin itself up across the line, which jams just as firmly. Either way one pin out of five is enough — press Use a key, drag a notch, and watch the red mark appear at the line. Locksmiths write a key as a string of depth numbers, its bitting, and cut it from that: 3,125 combinations for five pins at five depths, before you throw out the ones a machine cannot cut. Real keys obey a rule called MACS — a limit on how much neighbouring cuts may differ, because too steep a slope between two notches makes the key snag on the pin as it goes in. The locks here obey it too.

The idea is old. Wooden pin locks with the same principle were used in Egypt and Assyria nearly four thousand years ago; a wooden key the size of a forearm lifted wooden pins out of a bolt. The modern version is Linus Yale Jr.'s, patented in the United States in 1861 and refined in 1865 into the small flat key with a serrated edge that we all carry. It has barely changed since. The lock you touch every day is a Victorian invention held to tolerances of a thousandth of an inch.

Those tolerances are the flaw. If the five holes were drilled in perfect line, a lock would be unpickable, because a turning force would meet all five driver pins at once. They are not perfect. One chamber is always fractionally further round than the others, so when you put a light turning pressure on the plug, the plug rotates a hair and traps exactly one pin — the binding pin. Lift it and it feels stiff and gritty, because it is carrying the whole load. Lift any other and it springs straight back. Keep going and the binding pin's driver rises past the line; the plug turns another hair, the driver lands on the ledge of the plug and stays there, held by friction and a rotation of about a degree. Now the next-worst chamber binds. Five times, and the lock is open. That is all lock picking is: the lock tells you, one pin at a time, which pin it wants.

Lift too far and the key pin goes above the line and gets caught there — oversetting. There is no fix but to release the plug and drop everything, which is why picking is done with the lightest pressure anyone can manage. The window you are aiming for is the clearance between plug and shell, a few thousandths of an inch; it is exaggerated here, and it is the reason a picker works by feel and not by sight.

Rake it throws away all the finesse: a rake is a wavy bit of steel scrubbed in and out of the keyway, jiggling pins at random under light tension. It has no idea which pin binds. It does not need one — pins that pass through their window while bound simply stay set, and enough random passes will stumble over all five. Watch the counter: raking a five-pin lock usually takes a few dozen jiggles and a couple of oversets, and on a cheap lock it takes seconds. This is the honest reason a five-pin lock with plain pins is not a serious security device, and why better locks add spool or serrated pins, whose waists punish exactly that behaviour by giving false feedback and throwing set pins back down.

Two footnotes on the same geometry. A bump key is cut to the deepest depth at every position; struck sharply, it flicks all the key pins upward at once, and for a few milliseconds, while the driver pins are in the air above the line and the key pins have dropped back, a light turning pressure opens the lock. And a master-keyed lock has a second, thinner wafer in each stack, so that each pin has two heights that work — which is convenient for a building manager and terrible for security: given one ordinary key and a file, the master key can be worked out one pin at a time, a hole Matt Blaze described publicly in 2003 and locksmiths had kept quiet for a century.

Everything is simulated in your browser; nothing is uploaded. The lock's bitting and binding order come from a seeded generator — ?seed= in the address bar reproduces one exactly. The rake is given a one-in-three chance of overshooting a pin it could have set, which is about what a scrubbing motion earns you. Tab to the lock, then ← → to choose a pin, ↑ ↓ to lift it, Space for turning pressure, R to rake, N for a new lock.