All mainstream credit card numbers obey a mathematical trick designed to catch the most common typos. It’s called the Luhn ...

Your card number is carefully structured. The very first digit is what’s known as the major industry identifier. A 4 means ...

This catches all common errors, but can result in a check digit of 10 which is represented by an X. There are other even more robust algorithms such as Damm, Verhoeff, and Luhn.

Check digits aren’t infallible – some algorithms can’t tell the difference between sequences like 1 and 00001, for example; others will catch incorrect digits, but miss swapped pairs.