The CIE defines chromaticity coordinates as the ratio of each of a set of three tristimulus values to their sum. As shown here, for two-equation systems such as 1964 (u,v), there is always a tristimulus coordinate system that allows this definition to work. However, to include systems like that of MacLeod and Boynton in which three equations define chromaticity, an extension of the CIE definition would require a linear transformation that follows the ratio process. (c) 2013 Wiley Periodicals, Inc. Col Res Appl, 39, 317-318, 2014; Published Online 20 December 2013 in Wiley Online Library (). DOI 10.1002/col.21863