Airline tickets
Airline tickets have a ten-digit serial number followed by a check digit. This check digit is calculated using the modular scheme discussed above, with modulus $N=7$.
For example, a ticket might have serial number 3387972544. Since this number equals $5(mbox{mod} 7)$, the check digit is 5. The number is printed on the ticket with the check digit: 33879725445.
This is quite a primitive method. Is it really any good at detecting errors? From the list of relative frequencies of errors, we can see that far the most important errors to detect are single-digit errors and transpositions of adjacent digits. Let’s see how well this modular scheme does in these cases.