Os primeiros 10 códigos de barras upc-a

The UPC exists because American supermarkets were drowning in price stickers. The underlying idea was much older: Norman Joseph Woodland and Bernard Silver filed a patent application on 20 October 1949 and received US Patent 2,612,994 in October 1952, for a symbol made of concentric rings. Nothing came of it. The patent's seventeen-year term ran out before the laser and the integrated circuit made reading such a symbol cheap enough to matter.

At the end of the 1960s the American grocery trade set up an ad hoc committee to settle on a single product code. Several companies submitted symbols. IBM's entry was designed by George Laurer — Woodland was by then an IBM employee himself — and replaced the rings with vertical bars, which degraded gracefully when ink smeared along the direction of printing instead of being destroyed by it. On 3 April 1973 the committee adopted it, and the Uniform Code Council was founded the following year to hand out the numbers.

The first live scan came at the Marsh Supermarket in Troy, Ohio, at 8:01 on the morning of 26 June 1974: Sharon Buchanan rang up a 10-pack of Wrigley's Juicy Fruit gum for Clyde Dawson, Marsh's director of research and development. The 67-cent pack went to the Smithsonian.

Adoption was slow at first — scanners were expensive, and consumer groups objected to stores stripping the printed price off each item — and then it became total. EAN International and the Uniform Code Council, which had grown up on either side of the Atlantic, converged on a single name, GS1, from 2005. In GS1's vocabulary the twelve-digit UPC-A is GTIN-12: a zero-padded special case of the thirteen-digit international number rather than a system of its own.

It also picked up a conspiracy theory on the way. Mary Stewart Relfe's 1982 book The New Money System 666 popularised the claim that the three pairs of long guard bars encode 666. Laurer answered it himself: the guard patterns frame the symbol and let the scanner's gain circuitry adjust to the contrast of the particular label. They are not digit encodings at all.

Propriedades principais

• A UPC-A is twelve digits: a number system digit, a five-digit manufacturer code, a five-digit item code and one check digit.
• The check digit makes the weighted sum of all twelve digits a multiple of 10, with weights of 1 and 3 assigned alternately from the right: the check digit weighs 1, the digit to its left 3, then 1, 3 and so on. On a twelve-digit code that leaves weight 3 on the leading digit.
• Prepending a zero turns a UPC-A into a valid EAN-13 with exactly the same check digit, because the weights are anchored at the right-hand end and a leading zero adds nothing to the sum.
• Number system 2 is for variable-weight items such as meat and produce, where the trailing digits carry a weight or a price rather than a product identity.
• Number system 4 is reserved for local store and warehouse use, including loyalty cards and store coupons; 3 identifies drugs by National Drug Code and 5 identifies coupons. The remaining values — 0, 1 and 6 through 9 — cover ordinary retail items.
• UPC-E is an eight-digit printed form of a GTIN-12 that suppresses four zeros in codes with a suitable company prefix. The scanner expands it back to the full twelve digits before anything looks it up.
• The guard patterns at the left, centre and right of the symbol are 101, 01010 and 101. They are framing and gain-setting patterns rather than digits: the left-hand encoding of a 6 is 0101111, which is a different thing entirely.
• ISO/IEC 15420 specifies the symbology shared by UPC-A, UPC-E, EAN-13 and EAN-8. What the digits are allowed to mean is defined separately, in the GS1 General Specifications.