How is a check digit used as a validation check?

A check digit, typically the last digit of a number, can be computed from the other digits. For example, if you have an account number 1234, the bank might add a 5th digit of 0 because 1+2+3+4=10 and the low digit of 10 is zero. So they would tell you your account number is 12340.

Then if you type it in by mistake as 12350 or 12347 or some such, a web page could immediately know it was wrong before looking it up in the database.

If you typed in 33310 by mistake, it would think the check digit was correct -- so you can see it is good for saying what is probably wrong, but not what is RIGHT.

Real check digits will detect swapping of two digits (which the sum above does not). Also, today a check digit is less important than it was in 1980 as it is quick to actually look it up and see if it is a valid account number without using a check digit at all.