the letter "A" is a check. This method, like the ISBN method ,yields a 100% detection rate for both single-digit and transposition errors , but utilizes two formats for numbers. Notice that there would be nothing lost if the weighting vector began with 8 instead of 2 and there would be a slight gain since errors of the form a1a2....a7a8>>a7a2....a1a8 would be detectable.