Each check digit is computed by multiplying the other digits by weights (for example 9 down to 2, left to right), adding them up and applying a modulus, almost always 11. In the most common rule, if the remainder of the division by 11 is 0 or 1 the digit is 0, otherwise it is 11 minus the remainder. Variations exist: AL and RN multiply the sum by 10, RR uses modulus 9, BA uses modulus 10 or 11 depending on the first or second digit, and MG adds up the digits of the products.
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