How one extra digit on your bank card catches your typos

The last digit of your bank card number isn't part of your account. It's a check digit, computed from the other fifteen by a recipe an IBM engineer patented in the 1950s.

Double every other digit, add everything up, and a valid number always lands on a multiple of ten. That simple rule catches every single mistyped digit and almost every swap of two neighbours. Break a number yourself and find the one swap it can't see.