A lever tumbler lock is a type of lock that uses a set of levers to prevent the bolt from moving in the lock. In the simplest of these, lifting the tumbler above a certain height will allow the bolt to slide past. The number of levers may vary, but is usually an odd number for a lock that can be opened from each side of the door in order to provide symmetry. A minimum number of levers may be specified to provide an anticipated level of security.

‘Double acting’ lever tumbler locks were invented in 1778 by Robert Barron of England.These required the lever to be lifted to a certain height by having a slot cut in the lever, so lifting the lever too far was as bad as not lifting the lever far enough. This type of lock is still currently used today.

Lever locks generally use a bitted key. Some locks used on safes use a double-bitted key, as do some door locks of a type often used in Southern and Eastern Europe.

[Wikipedia}