The bit of a key is the part that actually engages the locking mechanism of a lock. (For example the tumblers in a pin tumbler lock.) The exact geometry of modern keys is usually described by a code system. This is referred to as the bitting. The bitting instructs a locksmith how to cut a certain key, to replace a lost key or make an additional copy. The bitting is usually a series of integers (e.g. 372164) that is usually translated from a key code chart or from a bitting code list to settings on specially designed key machines. In many code systems each digit in the bitting corresponds to a certain location on the key blank where a cut or notch is to be made and also indicates the necessary depth of the cut. Many lock companies use their own proprietary code system. Depending on the maker, the bitting sequence can be from bow-to-tip (the bow being the larger, handle portion of the key), or can be from tip-to-bow as is in the case of Best Locking Systems and Assa Abloy. A smaller number is typically a shallower cut on the key, but not always. Assa bitting codes are reversed, where the higher the digit, the shallower the cut. One American manufacturer, Eagle Lock Company, used letters exclusively for bitting codes.

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