A system used by most credit card companies to assign a check digit to the card number. The first 15 digits of a credit card number are assigned by the issuing bank: the 16th is the check digit.

To obtain the value, 3 different values are required:

• Twice the sum of the digits in the odd positions.
• The sum of the digits in the even positions.
• The number of digits in odd positions with a value greater than 4.

These 3 values are then added to give a fourth value. The check digit is the number that needs to be added to this fourth value to bring it up to the next power of 10.

An example: Consider the number 3141 5926 5358 979. Our three values are (3 + 4 + 5 + 2 + 5 + 5 + 9 + 9) x 2 = 84 ; 1 + 1 + 9 + 6 + 3 + 8 + 7 = 35 and 5. Adding these gives 124. The next power of 10 is 130, so the check digit is 6.

The Codabar method is one of the most efficient check digit generation techniques around: it will pick up all single digit errors and many common errors such as switching two adjacent digits.