Wherever you buy clothes or books in market, you can find the seller scan barcode labels on the products. And they can get many useful information such as price, quantity, item and so on from scanning the labels.

The Universal Product Code (UPC) is a barcode symbology. For example, UPC label is a special type of barcode labels that is widely used in North America, the United Kingdom, Australia, New Zealand and in other countries for tracking trade items in stores. Its most common form, the UPC-A, consists of 12 numerical digits, which are uniquely assigned to each trade item. Along with the related EAN barcode, the UPC is the only barcode allowed for scanning trade items at the point of sale, per GS1 standards.[1] UPC data structures are a component of GTINs (Global Trade Item Numbers). All of these data structures follow the global GS1 standards.

Each UPC-A barcode consists of a scannable strip of black bars and white spaces, above a sequence of 12 numerical digits and you can see such common barcode labels on the hang tag when buying clothing. No letters, characters, or other content of any kind may appear on a standard UPC-A barcode. The digits and bars maintain a one-to-one correspondence - in other words, there is only one way to represent each 12-digit number visually, and there is only one way to represent each visual barcode numerically.

The scannable area of every UPC-A barcode follows the pattern SLLLLLLMRRRRRRE, where the S (start), M (middle), and E (end) guard bars are represented exactly the same on every UPC and the L (left) and R (right) sections collectively represent the 12 numerical digits that make each UPC unique. The first digit L is the prefix. The last digit R is an error correcting check digit, allowing some errors in scanning or manual entry to be detected. The non-numerical identifiers, the guard bars, separate the two groups of six digits and establish the timing.