Lever Tumbler Locks
This style of lock is a variation on the principle of the first locks ever used - a bolt is released when the corresponding key fits the mechanism, moving the levers to the correct heights and allowing the bolt to slide out smoothly. These may also be known as lever mortice sashlocks and deadlocks - available as 2 and 3 lever locks or 5 lever locks.
In 1778 Robert Barron invented the Double Acting Lever Lock that used several levers instead of the previous two. The original locks only had two levers because the first levers used were so large that more than two would not have been practical for use in a normal wooden door. Only having two levers made these locks incredibly easy to pick. Once advances in technology enabled the levers to become thinner, up to six would be used in one lock, thus greatly increasing security.
The new thin levers greatly improved the security of the lock but unfortunately the cunningness of thieves moved with the times as well, so the lock wasn't completely secure.
How does a lever tumbler lock work?
A lever tumbler lock features a series of levers, normally three or five, that are pushed into the correct height to release a bolt. They typically use a bitted key which has a flat surface to push the levers into the correct position to allow the door to open.
Problems with this kind of lock
In order for these locks to be totally secure they must be manufactured perfectly. If the bolt and lever have slightly rounded corners they can be manipulated into a position that will force the lock open. Since few locks at the time were manufactured perfectly, this left a gap where thieves could pick the lock by testing the springs and manipulating them into the notches on the bolt which would, with some applied pressure, cause the bolt to be released. This is why future generations of locks started to incorporate safety mechanisms into their production.
This style of lock mechanism is still used in locks today, though with the more advanced security and complex lever systems developed by well known lock companies such as Chubb.