For 2 digit checks it's often mod 97 (largest prime < 100) because it can detect all single-digit errors and most adjacent digit transpositions. Used in IBAN b

For 2 digit checks it's often mod 97 (largest prime < 100) because it can detect all single-digit errors and most adjacent digit transpositions. Used in IBAN bank account numbers, EU VAT numbers (UK, FR, BE), etc