A Pin Tumbler Lock is a lock mechanism that uses pins. These pins are different in length to prevent the lock opening without the correct key. They are most commonly used in cylinder locks but can also be found in tubular tumbler locks or radial locks. It has an outer casing which has a cylindrical hole in which the plug is housed which must be rotated to open the lock. The basic principles may date back as far as 2000 BC in Egypt. It consisted of a wooden post fixed to a door and a horizontal bolt which slid into the post. This bolt had vertical openings into which was fitted with a series of pins. When they were lifted to a certain height using a special key which allowed the bolt to move and unlock the door. The modern day pin tumbler lock however was invented by Mr Linus Yale in 1848. In 1861, his son Linus Yale junior patented a smaller flat key with serrated edges as well as pins of different lengths within the lock itself and it is this design which remains in current day use.