How does a key open only its own lock?
Try a few keys. Why does only one of them turn?
Your key
Slide a key in.
Springs push the pins down into the slot where the key goes. Pick a key, then drag it into the lock or press Insert. Watch the pins ride up and down over its bumps.
Picked: Your key. It’s out of the lock. Slide it in, or press Insert.
A simple pin tumbler lock with five pins, like many real ones (some have six). The sizes are stretched so they’re easy to see. The circle in the top left corner is the front of the cylinder, where the key goes in.
Why the cuts have to match.
The bottom pins have different lengths. A deep cut leaves room for a longer pin. A shallow cut lifts a shorter pin higher. With the right key all the way in, every split ends up on the same gap line. Another lock can have a different order of pin lengths, so it needs different cuts.
Lift a pin too little and the top pin still crosses the gap line. Lift it too far and the bottom pin crosses instead. Either one pins the cylinder to the case. All the cuts have to match, not just most of them.
Your key probably isn’t one of a kind.
A real lock like this usually has five or six pins, and each pin has a handful of depths to pick from. That adds up to thousands, maybe tens of thousands, of different keys for one kind of lock. There are far more locks than that out there, so other keys with your exact cuts almost certainly exist. They’re probably just nowhere near yours.
Why the springs matter.
Take the key away and the springs push every top pin (the driver pin) down across the gap line. Now the case and the cylinder are pinned together, so nothing can turn.
A lock is shut until something lifts each pin to exactly the right height. That’s the whole job of the key.
No key: every top pin crosses the gap line.
Talk like you’ve cut a key
Pin
A small rod in the lock. Each one comes in two pieces: a bottom pin that rests on the key and a top pin that a spring pushes down. Locksmiths call them the key pin and the driver pin. Where they meet is the split.
“Every pin has to sit just right.”Cylinder
The round part that turns when the right key goes in. Locksmiths also call it the plug.
“Nothing blocks the cylinder, so it turns.”Gap line
The thin line where the cylinder meets the case around it. Locksmiths call it the shear line.
“Every split lines up with the gap line.”