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Guarding and protecting families and possessions is an ancient practice that has led throughout the centuries to widespread use of various types of locking devices. Locks have been, and still are, used to secure residences, commercial buildings, and other items, such as automobiles and safe deposit boxes. The oldest known lock and key device, which dates to about 4,000 years ago and is quite large, was found in the ruins of the Khorsabad palace near the biblical city of Nineveh. That lock was of the wooden pin-tumbler type, a form that was widely used in Egypt and also found in Japan, Norway, and the Faeroe Islands (and is still being used in parts of the Near East today). The modern Yale cylinder lock is actually based on this Egyptian pin-tumbler mechanism.

Roman locksmiths introduced metal locks (made primarily of iron and bronze), padlocks, and warded locks, which are made with varied projections around the keyhole. They also designed keys fashioned as rings so they could be carried easily, supposedly because togas had no pockets. Another important Roman contribution was the craft of making small locks to be used with tiny keys. Elaborate and intricate decorative surface designs introduced by craftspeople in Germany and France during the Middle Ages transformed locks into works of art; however, these locks showed little improvement in safety and security.

Special machines allow locksmiths to create and duplicate keys for any lock. The history of the industry includes a list of locksmiths who contributed to design developments. In 1778, the Englishman Robert Barron patented a lever lock with double-acting tumblers. Just 40 years later, his fellow countryman, Jeremiah Chubb, improved on the reliability of the lever lock by incorporating a detector in its mechanism. Meanwhile, in 1784, Joseph Bramah, also from England, had introduced his innovative Bramah lock and key, which was to remain "unpickable" for more than 50 years. In 1851, Robert Newell of New York exhibited his Parautoptic lock, which reputedly remains unpicked to this day.

In 1848, Linus Yale, of the United States, patented a pin-tumbler lock, from which his son, Linus Jr., devised the Yale cylinder lock during the 1860s. James Sargent of Rochester, N.Y., adapted an earlier Scots patent in 1873 for a lock that incorporated a clock, allowing vaults and safes to be opened only at preset times. Other lock experts experimented with the letter-lock until the keyless combination device was perfected.

Since the 1800s, many other types of locks have been devised for specific purposes. However, the most reputable and the most commonly used of today's nonelectronic locks are direct descendants of the original Yale cylinder, the Bramah, and combination devices.

The advent of new technology in the middle to late 20th century has led to increased and widespread use of electronic-access control devices. Such security equipment is based on fundamental electronic wiring and utilizes any of a variety of mechanisms, such as plastic credit card-shaped "keys" with magnetic code strips or electronic button-coded doorknobs. Electronic-access control devices have replaced manually operated locks in many circumstances, from large building complexes to automobile doors. New lock technology has resulted from the demands by security-conscious citizenry for complicated, sophisticated locks. Because such devices require skilled and knowledgeable care, some say that locksmiths have never had it so good.