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 original North American Universal Product Code (UPC), which dates to 1971, used a set of two dark (usually black) and two light (usually white) bars of specified thicknesses to represent 12 numbers, but beginning in 2005 the Uniform Code Council, now known as GS1 US, adopted the similar European Article Numbering Code (EAN), which encodes 13 numbers and had become the international standard. The standards for the international product bar code system are managed by GS1, formerly known as EAN International, which is based in Brussels. 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.

An emerging technology, radio-frequency identification (RFID), could supplant the bar code in most applications. The newer radio-based devices overcome many of the limitations inherent in the bar code's optical technology.