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Yet more difficult to pick is the cylinder lock which Linus Yale invented in 1848. This contains a cylindrical plug into which fits a small, flat, notched key. The cylinder also contains small holes (usually five) bored in a line along the key slot. In those holes are small, loose pins. When the proper key is in the slot the pins fit into the notches and are flush with the surface of the plug. This permits the turning of the plug (and key) and the throwing of the bolt. If the improper key is used, plug pins are pushed up into the barrel of the lock and the plug kept from turning. Or loose pins in the barrel may drop into the plug holes. This also prevents turning.*
*An Egyptian drawbar lock, dug from the Palace of Khorsabad at Nineveh, was operated 4,000 years ago by such loose pins. The huge wooden key which raised the pins was doubtless like Isaiah's, who declared: "And the key of the House of David will I lay upon his shoulder." (Isaiah XXII, 22)