The model year is encoded in position 10 of North American VIN codes. Besides the three letters that are not allowed in the VIN itself (I, O and Q), the letters U and Z and the digit 0 are not used for the year code. Note that the year code can be the calendar year in which a vehicle is built, or a model or type year allocated by the manufacturer. The year 1980 was encoded by some manufacturers, especially General Motors and Chrysler, as "A" (since the 17-digit VIN wasn't mandatory until 1981, and the "A" or zero was in the manufacturer's pre-1981 placement in the VIN), yet Ford and AMC still used a zero for 1980. Subsequent years increment through the allowed letters, so that "Y" represents the year 2000. 2001 through 2009 are encoded as the digits 1 through 9, and subsequent years are encoded as "A", "B", "C", etc.