Check Digit
Position 9 of a US- or China-spec VIN; a calculated value used to detect typos and tampering.
The check digit is the single character at position 9 of a VIN issued in the United States, Canada, Mexico, China, and a handful of other regulated markets. It is not arbitrary: it’s computed from the other 16 characters using the SAE J272 algorithm (transliterate each letter to a number, multiply by a position-specific weight, sum, then take the result modulo 11; if the remainder is 10, the check digit is the letter X).
The point of the check digit is to detect the most common kinds of VIN errors: a single mistyped character or two characters swapped will almost always change the calculated check digit, so a verification tool can flag the VIN as likely-corrupted before the user wastes time looking up a number that doesn’t exist.
Vehicles built outside the US/China regulatory umbrella — many European-market BMWs, Mercedes and VWs sold within Europe, for example — are not required to follow the J272 calculation, so a check-digit mismatch on a European VIN doesn’t necessarily indicate a fake. Cross-reference the auction record before drawing any conclusions.