How A Lock Works

The modern “pin and tumbler” lock was invented by Linus Yale Jr in 1861, and today most locks work on the same principle. A tumbler (known as a ‘plug’ – the yellow circle in the diagram) sits inside the main lock housing (known as the ’shell’ – the beige background in the diagram). The plug has a slot for the key to be inserted (known as the ‘keyway’, shown in white), and the plug rotates to activate the locking mechanism. The interface between the plug and the shell is known as the ’shear line’.

Along the length of the plug are a series of pin chambers. Each pin chamber contains two sprung pins (known as a ‘pin stack’). The top pin is known as the ‘driver pin’ (shown in blue), and the lower pin is known as the ‘key pin’ (shown in red). The driver pins prevent the plug from rotating until a key is inserted which will move all the driver pins clear of the plug. Once all driver pins are in the chamber above the shear line (known as ‘the bible’), the plug can rotate freely.