Supermarket check-out counter showing bar-code reader in use. Bar codes consist of a row of parallel lines of varying thickness that permit fast error-free entry of data into a central computer. In this retail application the scanner (the light pen) is fixed, and the supermarket products with their individual codes pass along a conveyor and past the red light of the scanner, which records the bar code; details of date, unit cost, product name and other stock control data are inputted into the computer to be used for future accounting and stock control purposes.