Why did Robert Barron’s lock require each tumbler to be raised to a different height?

Robert Barron’s 1778 patent for a double-acting tumbler lock was a significant advancement in security because it required each tumbler—a lever or pawl—to be raised to a specific, precise height to clear a slot in the bolt.1

By requiring the key to lift each of the two tumblers by a different amount, the Barron lock prevented the bolt from being moved until the tumblers were correctly positioned. This design ensured that the bolt could only be shot once the tumblers were cleared from the slot, establishing a fundamental principle that remains the basis for all modern lever locks.1