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bar code, computer coding system that uses a printed pattern of lines or bars to identify products, mail and packages, customer accounts, and the like. Bar codes are read by optically scanning the printed pattern and using a computer program to decode the pattern. In a linear bar code system, the code itself contains no information about the item to which it is assigned but represents a string of identifying numbers or letters. When the code is read by an optical scanner linked to a computer, the computer can provide and record information about the item, such as its price or the quantity sold, from and to databases. The North American Universal Product Code (UPC) uses a set of two dark (usually black) and two light (usually white) bars of specified thicknesses to represent numbers. The dark bars may be from one to three units wide and the light bars from one to four units. For registration purposes two one-unit dark bars are placed at each end and in the middle. Each item is assigned a unique numeric code, which is printed as a bar code on the item's packaging. So-called two-dimensional (2D) bar codes permit the encoding of information about an item in addition to an identifying code. In a 2D bar code, two axes, or directions, are used for recording and reading the codes and the bar size is reduced, increasing the space available for data in the way that a column of words improves on a column of letters. Some 2D codes do not use bars at all, such as the United Parcel Service's hexagon-based Maxicode.
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