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A key that is bitted to the wrong depth in even one pin position will not operate the lock. 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. Figure 2. Pin tumbler lock with a correct key inserted. 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. 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. Picks probe and lift the individual pin tumblers through the keyway, while torque tools control the degree and force of plug rotation. 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.

You may hear a faint "click." This is because you've pushed the cut (between the bottom and the top pin) up to exactly the shear line. Note the border between the plug and shell, which forms the shear line, what is billiards and the cuts in each pin stack resting within the plug. Right: Side view, with part of the shell and plug cut away to expose the six pin stacks. 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. Rotation of the plug within the shell operates the locking mechanism. 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.

Another style of torque tool has two "prongs" that fit in the top and bottom of the keyway, with a cutout between them for the pick. Both the pick and the torque tool also amplify and transmit feedback about the state of the lock back to their user. The proper pick and torque tool selection depend on the shape of the keyway, the features of the lock, the picking technique, and the individual preferences of the user. Left: Cylinder face, the lock's "user interface." Note the keyway, which is cut into the plug, which in turn sits inside the shell. Right: With all of the cuts at the shear line, the plug can rotate freely within the shell. Eventually, you'll reach a point where the plug will turn ever so slightly and the pin will not go up any farther. 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. In particular note that because the pins are slightly out of alignment, as the plug is turned gently, only the pin stack that is most out of alignment actually prevents further rotation.

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. Pin tumbler lock picking consists of raising the cuts on each pin stack to the shear line, one by one, until the plug turns freely. 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. There is a tradeoff, of course, since a longer handle may be difficult to maneuver around obstacles. The pick must be strong enough to resist bending or breaking while lifting pins, yet the shaft must be small and thin enough to maneuver freely around the keyway without disturbing other pins. If you're having trouble, you may be pushing adjacent pins up past the shear line as you pick a pin, causing them to be overset even before they start to bind.