The seam is the secret
A lock never checks your key. It checks five gaps. The plug turns the instant every gap between a gold pin and a red pin sits on the shear line, whatever put them there.
A field guide to the insides of things
How things work, with nothing skipped.
Each explainer takes one ordinary object apart. Slide it, turn it, slow it down and watch every piece do its job, until you could explain it to someone else.
Start with the lockExplainer No. 1
The pin-tumbler lock
Most front-door locks run on a single trick: a few stacks of tiny pins, and one line they all have to agree on. Step through it below, or grab the sliders and play.
Key
Locked
Pick a depth for each cut: 0 is shallow, 5 is deep. Get all five gaps onto the shear line.
Every moving part
Point at a part to find it in the drawings.
- Plug×1
The cylinder that turns. The keyway runs through it, and its back end turns the cam that works the bolt.
- Key×1
Five cuts, each one a height for one stack. All a key ever does is lift.
- Key pins×5
The gold bottom pins. Each is cut to a different length to match a different cut depth.
- Driver pins×5
The red top pins, usually all the same length. They're the ones that actually block the plug.
- Springs×5
Push each stack back down, so the lock re-locks itself the moment the key comes out.
- Housing
The fixed shell around the plug. It holds the top half of every pin hole.
- Shear line
Not a part at all: the seam where plug meets housing. Every gap has to land exactly here.
17moving parts, counting the key
5pin stacks
6depths per cut
7,776possible keys (6⁵)
Why the key is stuck while it's turned
To come out, the key has to push each gold pin up over its ridges. Once the plug turns, there's solid housing above those pins instead of a hole, so they can't rise and the key can't move. Turn back to the start and it slides right out.
Why close isn't good enough
On common house keys, neighboring cut depths are only about 0.4 to 0.6 mm apart. One cut off by a single step leaves one red pin straddling the seam, and one pin is all it takes.
Why lock picking works
No lock is machined perfectly, so the pin holes are a hair out of line. Turn the plug gently and one pin binds before the others. Lift it to the line and it stays put; then the next one binds. The drawing above has perfectly aligned holes, so it can't show this.
Older than it looks
Wooden locks in ancient Egypt may have used the same pin idea about 4,000 years ago. Linus Yale Sr. built the modern pin-tumbler lock in 1848, and in 1861 his son, Linus Yale Jr., patented the small, flat key with a serrated edge.
More explainers
Take something else apart
- No. 2 · newThe toilet flushA siphon, a float and a flapper.
- No. 3 · plannedThe mechanical watchHow a spring turns into seconds.
- No. 4 · plannedThe refrigeratorMoving heat uphill.
- No. 5 · plannedThe bicycle freewheelWhy the pedals can stop and the wheel keeps going.
- No. 6 · plannedThe zipperTwo rows of teeth and one clever slider.