I think "relatively prime" is the term for what we're after -- one or both of the two might not be a prime number, but if they don't have a common factor besides 1, then you get the full number of potential skid patches. For example, my fixed-gear uses a 45/16 ratio in the spring/summer/fall and neither sprocket has a prime number of teeth, but I still get 16 skid patches. (Not much use for me since I almost never skid, though. )