Explainer #2
How does a pin tumbler lock work?
Inside the lock, a cylinder called the plug is held still by stacks of spring-loaded pins that reach across the thin gap between the plug and the housing, the shear line. The cuts on the right key lift each stack to exactly the height where the split between its two pins sits on that line. When every gap lines up, nothing bridges the shear line, so the plug can turn and takes the cam that moves the bolt with it.
The principle
Locking by blocking a gap. Each pin chamber holds a key pin that touches the key, a driver pin above it and a spring. With no key in the lock, the springs push the driver pins down across the shear line, pinning the plug to the housing like bolts through two plates.
The key pins all have different lengths, and each cut on the key is matched to its pin. The right key lifts every stack by exactly the right amount. Too little, and the driver pin still crosses the line. Too much, and the key pin crosses it instead. Either way, the plug stays put.
With five pins and several possible cut depths per pin, a lock can have tens of thousands of different keys. The basic idea is ancient (wooden pin locks were used in Egypt thousands of years ago), and Linus Yale Sr. and Jr. developed the modern pin tumbler cylinder with a flat key in the mid-1800s.
Common myth
The key turns the bolt
The key doesn’t push or turn the bolt directly. All it does is line up the pins. Once the gaps are on the shear line, your twist turns the plug, and a cam or tailpiece at the back of the plug moves the latch or bolt. That’s why a key that fits the keyway perfectly but has the wrong cuts won’t even start to turn.How this video was made: Rendered 3D animation of a simplified five-pin model. Real locks have smaller pins, standard cut depths and a wavy keyway that stops the wrong key blanks from even going in, but the pins, springs and shear line work the same way.
