KeyIts cuts lift each pin stack to the right height.
PlugTurns inside the housing when every gap meets the shear line.
Cylinder housing (body)Holds the pin chambers above the plug.
Key pin (bottom pin)Pins of different lengths match the key's cuts.
Driver pin (top pin)Pushed down by the spring; blocks the plug unless lifted.
SpringPresses each pin stack down into the plug.
Shear lineWith the right key, every gap between pins lies on it.
Cam (tailpiece)Turns with the plug and works the bolt.
Rose (cylinder ring)A metal disc round the cylinder's outer face.
Front face 1
KeywayIts shape lets in only keys of the matching profile.
Deadbolt in the door 4
DeadboltMoved both ways by the key or the thumb turn.
Thumb turnThrows the bolt from inside without a key.
Forend (faceplate)Fixes the lock to the door edge.
Strike plate (striking plate)Reinforces the frame where the bolt enters.
How this list was compiled: Read 2026-09-26. (1) Wikipedia, Pin tumbler lock, https://en.wikipedia.org/wiki/Pin_tumbler_lock : the plug in a hole in the outer casing; the keyway at one end of the plug and a cam or lever at the other to retract a bolt; key pins of various lengths, rounded; spring-loaded driver pins above them; the bible, the upper part holding pins and springs; with the correct key the gaps between key pins and driver pins line up with the boundary of plug and bible, and the plug turns. (2) Master Locksmiths Association, Dictionary of locksmith terms, https://www.locksmiths.co.uk/locksmith-terminology-parts-of-locks/ : plug, cylinder and cylinder housing, pins, top pins (drivers) and springs, shear line, cam as a tongue fixed on the plug's end, keyhole often called the keyway, cylinder rose or ring, bolt, deadbolt moved by the key or a thumb turn inside, forend, striking plate, key with bow and blade. Licence: own work
The drawing: Drawn for this site by script, 2026-09-26. Licence: own work
How it works
These are the parts of a lock cylinder and the deadbolt it turns. The key lifts each stack of pins until every gap sits on the shear line, so the plug can turn and its cam throws the bolt.
Read the full guide
Inside the cylinder
The plug turns inside the cylinder housing. Five stacks of pins cross the line between them: a key pin below, a driver pin above and a spring on top. Without a key the driver pins block the plug; the right key lifts every gap to the shear line.
Parts of a door lockset
On a deadbolt the cam, or tailpiece, runs from the cylinder through the bolt mechanism to the thumb turn inside. Turn either one and the bolt slides out through the forend into the strike plate on the frame.
Questions and answers
What is the shear line in a lock?
The boundary of the plug with the housing around it. The plug can turn only when the gap in every pin stack sits exactly on that line, which is what the right key does.
What is the difference between a key pin and a driver pin?
The key pin is the lower pin, rounded where the key touches it, and its length matches one cut of the key. The driver pin sits above it, pushed down by the spring.
What does the thumb turn do?
It throws and withdraws the deadbolt from inside the door without a key, turning the same tailpiece that the key turns from outside.