Left: Cylinder face, the lock's "user interface." Note the keyway, which is cut into the plug, which in turn sits inside the shell. If this pin stack is slowly pushed up with torque applied to the plug, eventually its cut will reach the shear line and the plug will turn a bit more. Left: The correct key lifts the pin stacks to align the cuts at the shear line. These imperfections are very small -- as little as .0001 inches in some cases -- but they are what allow us to manipulate ("pick") locks open without using the correct key. It's much easier to learn each skill in isolation, using locks specifically set up for the purpose. In the locked state the plug is prevented from rotating by a set of movable pin stacks, typically under spring pressure, that protrude from holes in the top of the opening in the shell into corresponding holes drilled into the top of the plug.

In practice, of course, locks aren't perfect: the pin holes in the plug are slightly out of alignment with respect to the shell and the pins and pin holes are each of a slightly different diameter. In an ideal lock, all of the pin holes in the plug would be in perfect alignment with the corresponding holes in the shell, the centerline of the plug would be exactly parallel to that of the shell, and all of the pins would be exactly the same diameter. Note the border between the plug and shell, which forms the shear line, and the cuts in each pin stack resting within the plug. Around the circumference of the plug is a shell, which is fixed to the door or container. See Figure 2. The plug will be blocked from rotating if any pin stack is lifted either not far enough (with the cut still in the plug below the shear line) or too far (with the cut pushed above the shear line and into the shell); to rotate, all pin stacks must have a cut at the shear line. If you tried to rotate the plug of such a lock without a key in the keyway, the top pin segment of each pin stack would block the plug at exactly the same number of degrees of rotation; each pin stack would contribute equally to preventing the plug from turning.

When a key is inserted into the keyway slot at the front of the plug, the pin stacks are raised within the plug and shell. A key that is bitted to the wrong depth in even one pin position will not operate the lock. See Figure 1. (In practice, the cuts are produced by stacking pin segments of particular lengths, not by actually cutting the pins; hence the term "pin stack.") With no key in the lock, all the pin stack cuts rest within the plug. Figure 1. A pin tumbler lock cylinder. Two tools -- one for each function -- are used simultaneously when picking a lock. Lock picking is a core skill of the locksmithing trade and is also of value to those evaluating, investigating, and studying security systems. Many experienced locksmiths and expert lock pickers prefer "home made" tools to the commercial selections, especially for picking unusual and high security locks. Picking depends on weaknesses in the implementation of locks -- small manufacturing imperfections -- rather than fundamental, abstract design flaws that would be present no matter how carefully made the locks might be. Good tools are important, to be sure, but once a few basic tools are available the student of lock picking is usually better off investing in new locks on which to practice rather than in new picking tools.

The basic algorithm for picking locks is remarkably simple: - Apply a small amount of torque to the plug. Right: Side view, with part of the shell and plug cut away to expose the six pin stacks. Using the five or six pin lock, find a pick that lets you locate and lift each pin across its full range of motion without disturbing adjacent pins too much. The height (or cut depth) of a key under each pin stack position is called its bitting; the bitting of a key is the "secret" needed to open a lock. The plug can rotate freely only if the key lifts every pin stack's cut to align at the border between the plug and shell. Figure 2. Pin tumbler lock with a correct key inserted. The top pin of that pin stack will be trapped above the shear line, the bottom pin will fall freely, and now a new pin stack (the next most misaligned one) prevents further rotation. The top pin of the most misaligned pin stack becomes "pinched" at the shear line between the plug and the shell. The rest is just technique -- locating and recognizing the state of each pin stack, manipulating the pins, applying torque to the plug.

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