How do Barcldes Work

There are lots of types of barcode, but the common one we all see all the time is called UPC, the Universal Product Code.

Each digit is coded by two black and two white stripes. The stripes have widths of from 1 to 4 units, and the total width for each digit is always seven units.

This code is not quite all there is to it, however. Some digits of the barcode are reversed, so that they read right to left. Part of the code has black and white inverted, so black stripes are white and vice versa. This helps cut down errors and allows a computer to work out if the code was read from the wrong end when it was scanned.