Why is a VIN (Vehicle Identification Number) always 17 digits long?

I know this...lol...or some of it anyway. The first 3 numbers are to identify the mfg. the 4th-8th numbers tell about the vehicles attributes the 9th number is what's called a check digit but I don't know what that means. the 10th number is the vehicles year the 11th number is the plant it was made at.... and that's all I can tell you but it's a start!