Pin tumbler lock
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The pin tumbler lock (or Yale lock, after lock manufacturers Yale) is a lock mechanism that uses pins of varying lengths to prevent the lock from opening without the correct key. Pin tumblers are most commonly employed in cylinder locks, but may also be found in tubular pin tumbler locks (also known as radial locks).
The basic principles of the pin tumbler lock may date as far back as 4000 BC in Egypt; the lock consisted of a wooden post affixed to the door, and a horizontal bolt that slid into the post. The bolt had vertical openings into which a set of pins fit. These could be lifted, using a key, to a sufficient height to allow the bolt to move and unlock the door.
In 1805, the earliest patent for a double-acting pin tumbler lock was granted to American physician Abraham O. Stansbury in England. It was based on earlier Egyptian locks and Joseph Bramah's tubular pin tumbler lock. Two years later, Stansbury was granted a patent in the United States for his lock.
In 1848, Linus Yale, Sr. invented the modern pin-tumbler lock. In 1861, Linus Yale, Jr. was inspired by the original 1840s pin-tumbler lock designed by his father, thus inventing and patenting a smaller flat key with serrated edges as well as pins of varying lengths within the lock itself, the same design of the pin-tumbler lock which still remains in use today.
The pin tumbler is commonly used in cylinder locks. In this type of lock, an outer casing has a cylindrical hole in which the plug is housed. To open the lock, the plug must rotate.
The plug has a straight-shaped slot known as the keyway at one end to allow the key to enter the plug; the other end may have a cam or lever which activates a mechanism to retract a locking bolt. The keyway often has protruding ledges which serve to prevent the key pins from falling into the plug, and to make the lock more resistant to picking. A series of holes, typically five or six of them, are drilled vertically into the plug. These holes contain key pins of various lengths, which are rounded to permit the key to slide over them easily.
Above each key pin is a corresponding set of driver pins, which are spring-loaded. Simpler locks typically have only one driver pin for each key pin, but locks requiring multi-keyed entry, such as a group of locks having a master key, may have extra driver pins known as spacer pins. The outer casing has several vertical shafts, which hold the spring-loaded pins.
When the plug and outer casing are assembled, the pins are pushed down into the plug by the springs. The point where the plug and cylinder meet is called the shear point. With a key properly cut and inserted into the groove on the end of the plug, the pins will rise causing them to align exactly at the shear point. This allows the plug to rotate, thus opening the lock. When the key is not in the lock, the pins straddle the shear point, preventing the plug from rotating.
Master keying 
A master keyed lock is a variation of the pin tumbler lock that allows the lock to be opened with two (or more) different keys. This type is often used for doorlocks in commercial buildings with multiple tenants, such as office buildings, hotels, and storage facilities. Each tenant is given a key that only unlocks his own door, called the change key, but the second key is the master key, which unlocks all the doors, and is usually kept by the building manager, so he can enter any room in the building.
In a master keyed lock, each of the pin stacks in the lock has three pins in it instead of two. Between the driver pin and the key pin is a third pin called the spacer pin. Thus each pin line has two shear points, one where the driver and spacer pins meet, and one where the spacer and key pins meet. So the lock will open with two keys, one aligns the first set of shear points and the other aligns the second set of shear points. The locks are manufactured so one set of shear points is unique to each lock, while the second set is identical in all the locks. A more secure type of mechanism has two separate tumblers, each opened by one key.
More complicated master-key lock systems are also made, with two or more levels of master keying, so there can be subordinate master keys which open only certain subsets of the locks, and a top-level master key which opens all the locks.
See also 
- Cylinder lock
- Disc tumbler lock
- Wafer tumbler lock
- Tubular pin tumbler lock
- Key relevance
- Lock picking
- Magnetic-coded lock
- Locksmithing: From Apprentice to Master. McGraw-Hill Professional. p. 79.
- The Complete Book of Home, Site, and Office Security: Selecting, Installing, and Troubleshooting Systems and Devices. McGraw-Hill Professional. p. 11.
- The Geek Atlas: 128 Places Where Science and Technology Come Alive. O'Reilly Media, Inc. p. 445.
- "Inventor of the Week Archive". Massachusetts Institute of Technology.
- The pin tumbler invention at Yale company site
- Popular Science, November 1946 How Your Home Lock Works