The Yale lock, also known as a pin tumbler lock, was invented in 1848 by Linus Yale, Sr. Later improved by his son, the Yale name has become synonymous with that type of lock. The principles of the pin tumbler lock, however, have been around since ancient Egypt, but only 19th century technology made it practical for common use.
The basic design consists of a rotating cylinder tube, called the plug, linked to the locking mechanism. Around the plug is a shell, which is fixed to the door. Turning the plug within the shell operates the locking mechanism.
When locked, the plug is prevented from rotating by a set of movable pins that protrude from holes in the top of the opening in the shell into corresponding holes in the top of the plug. When locked, the pins are fully extended into the plug. The pins are typically under spring pressure; otherwise, a lock could be picked just by turning it upside down and letting the pins fall.
The Shear Line
The division between the shell and the plug is called the shear line. The only way a lock can open is if all the pin breaks are above the shear line; otherwise, they will physically prevent the lock from turning.
A correctly fitted key for a pin tumbler lock is adjusted to the individual lengths of the pins. When inserted, the ridges of the key, or bitting, raise the pins exactly to the level of the shear line. Too high or too low, and the pin will catch the edge of the shell.
The principles of the Yale lock are found in many other locks. European cylinders are essentially upside down pin tumbler locks. Some locks have two sets of pins, multiple shear lines, or differently shaped keys, but operate on the same principles.