The door lock
Why does only your key open your door?
The question
Your key and your neighbour's look almost the same. So why does only yours turn the lock?
Have a guess, then show the answer
The key doesn't push the lock open. Its cuts lift stacks of spring-loaded pins so that every gap between them lines up with the edge of the part that turns. Only then can it turn.
Play
Insert a key, then try to turn it
This is a lock cut in half from the side. Pick a key, insert it, and try to turn it. A pin across the shear line turns red. Then set the cut depths yourself and find the pattern that opens the lock.
No key: the springs push every driver pin down across the shear line, so the plug can’t turn. Insert a key.
What's happening
- 1
Inside the lock, a brass plug has to turn in its housing. The line where the plug meets the housing is called the shear line.
- 2
Stacks of pins stop it. Each stack is a key pin, sized to match a cut on the key, with a driver pin above it, pushed down by a spring.
- 3
With no key in, the springs push the driver pins down across the shear line, so the plug can't turn.
- 4
The right key's cuts lift every stack so the gap between its key pin and driver pin sits exactly on the shear line. Nothing crosses the line, so the plug turns.
- 5
A wrong key leaves at least one pin across the line, and the plug is stuck. Most front doors have five or six stacks.
Simplified, and sources
The drawing is a simplified cutaway with five stacks, the commonest count. The cut depths are made-up units from 0 to 4, not real sizes, and every driver pin is drawn the same length. Real locks also have parts not drawn here, such as the cam that works the bolt, and some have security pins. The key goes in and turns more slowly than a real one, and the small end view shows the plug turning, which the side cutaway can't. Lots of keys share the same cuts, so your key isn't one of a kind; it just doesn't match your neighbour's lock.