Regulations in the United States and Canada require that the 9th digit of the Vehicle Identification Number be a check digit. Its purpose is to provide a means for verifying the accuracy of the VIN transcription.
Doing their calculations gives us:
6, My check digit. Now the astute will have noted, dividing by 11 can give you 0 to 10 as remainders. If the remainder is 10 then an X is used as the check digit. As far as I can tell that's the only allowed letter.
Thanks to: Cliff Simpson for this information!