Three nines fine is a rating on the millesimal fineness scale. It describes the purity of a precious metal by indicating the number of parts per thousand of the pure metal are present. Therefore, 'three nines fine' indicates a fineness of 999 parts per thousand, or 99.9 percent of the pure metal present. In the U.S., 'three nines fine' is equivalent to 24 karat in the case of gold.