Most grocery products include an identifying Bar Code on their wrappers and many supermarkets now use these bar codes for totalling sales at the checkout, using a light pen to read the code.

The UPC (Universal Product Code) was introduced in America in 1973 and adapted to form EAN (European Article Code) in 1974. There are two versions of EAN - 13 digit and 8 digit, but we will deal with the 8 digit version. An example is shown below.

This version is used by stores such as Sainsbury or Boots to code their own label products. The Number is divided into three parts 00 34600 9 retailers' code product code check digit

The check digit is chosen so that

is exactly divisble by 10.

Another 8 digit EAN is shown opposite. It has left and right hand guard bars and centre bars. In between there are 8 bars of varying thickness. Each number is represented by a unique set of 2 bars and 2 spaces. As can be seen in the magnified version of 5, each number code is made up of 7 modules. We write 5 as 0110001 to indicate whether a module is blank (0) or black (1).

All left hand numbers start with 0 and end with 1, and only use a total of 3 or 5 black modules. Right hand numbers are the complement of the corresponding left hand code e.g. right hand 5 = 1001110.