Rather than being specifically chosen by the credit card company, the check digit is automatically determined by the Luhn algorithm based on the preceding numbers in the sequence. When users enter ...

Originally patented, the algorithm is now in the public domain and a Worldwide standard ISO/IEC 7812-1. Obviously, with just a single check digit, not all errors can be detected (there’s a one ...

That’s why the last digit in the string is called the “check digit,” which completes the algorithm so it is divisible by 10. It can quickly identify mistakes when numbers are manually ...