How did Joseph Bramah’s lock work?

Joseph Bramah’s lock, patented in 1784, operated on a unique principle distinct from the lever-based mechanisms of his contemporaries. Instead of using a traditional key to lift tumblers, the Bramah lock utilized a small, tubular metal key with narrow longitudinal slots cut into its end.1

When this key was inserted into the lock, it depressed a series of internal slides to specific depths determined by the slots on the key. The lock could only be opened—allowing the key to turn and the bolt to be thrown—when every slide was depressed to the exact required distance.1

Due to the high level of precision required for these intricate components, the lock was difficult and expensive to manufacture. To overcome this, Bramah and his assistant, Henry Maudslay, developed specialized machine tools to produce the parts, marking a significant early milestone in the history of mass production and the British machine-tool industry. The design was famously secure, remaining unpicked for over 50 years until 1851.21