The idea is that you can also just add up the digits of a number to 'cast out nines' (during this calculation you can also ignore any pair of digits that adds up to 9).

By the way, you can combine this with 'casting out tens', then you'll know if your result is accurate mod 90 (which is even less likely than it being accurate mod 10 or mod 9, if you've made a mistake).

This method uses all the digits in the number so that it is possible to detect an error in any position, not just in the least-significant digit.