The first locks were probably those used by the Chinese more than 4000 years ago, these were a very simple arrangement which required no key. It was the Egyptians who developed the lock to the degree that added security was given through the need for a key to operate the lock. The lock made rapid developments in medieval Britain, many along the lines of the cast iron padlock.
The most popular form of modern lock is the `Yale' lock, invented in the 1850's by the American L. Yale. This lock utilises a system of tumblers giving almost infinite combinations of key profile. As the key is inserted into the keyway, it pushes past tumblers which rise and fall. Once the key is fully inserted, the series of tumblers will be resting on the keys profile at varying heights. If each tumbler is at the correct height then the key may be rotated. This then withdraws the latch from the frame mounted retainer and the door may be opened.
The Yale lock is usually fitted with a manual catch which allows the operator to permanently lock the unit so that it may not be operated, even with the correct key.
