The fourth to eigth characters belong to the Vehicle Descriptor Section (VDS). These five characters are used by manufacturers to identify your car’s attributes. They'll tell you what size engine you have, what platform, transmission type, and body type.
The ninth character is the check digit (something unique to North America). It's used to verify VIN accuracy, and ensures that the VINs of any two vehicles produced within a 30 year period won't be identical. There's a lot of math that go into calculating how to use the check digit with your VIN, and as long as your Mustang doesn't have a suspicious history, you shouldn't have to worry about this digit.
The tenth character identifies the model year. For example:
- 1988 – J
- 1989 – K
- 2000 – Y
- 2001 – 1
- 2002 – 2
The model year identifer starts at 1980 (when VINs were standarized) and started at the letter "A". After "Y" in 2000, the model year identifiers went to numbers starting with "1" in 2001. The number identifiers stopped at "9" and restarted at "A" in 2010.
The eleventh character identifies the assembly plant where your Mustang was built.
The twelfth to seventeenth characters belong to the Vehicle Identifier Section (VIS). This number is usually a sequential number. If your Mustang's VIS is 123456, the next Mustang off the assembly line would be 123457. Sometimes the first digit will designate a specific, optional trim line. The last five have to be numeric however.
If you want a simple way to decode your VIN quicky, check out this useful tool.