It's still amazing to me that so many sites have no idea how to make a simple form usable.
Basically it's a method used by credit card companies for decades now to detect errors in card numbers without having to look up the actual account.A lot of forms set the of the field to 16 characters which means that you can type most of your number - using spaces or dashes to separate blocks of numbers - before running out of space and having to go back and remove the spaces before entering the rest. Others will let you enter the number with extra characters, but then send you back an error message when the form is submitted. Let's see how we can do it better: What have we done then?: When the form is submitted the credit card input will be numbers only which is handy for processing, but you shouldn't force the user to do the work for you.The sum of all the digits in the third row is 67 x.The check digit (x) is obtained by computing the sum of the non-check digits then computing 9 times that value modulo 10 (in equation form, ((67 × 9) mod 10)).