See the introduction to color matching functions. Chromaticity coordinates are simply the ratio of the intensities of the three primary lights that appear identical to a series of monochromatic lights of equal energy traversing the visible spectrum.
The chromaticity coordinates, , and , are related to
the CMFs, , and , by:
(and similarly for , and and , and ).
A diagram in which any one of the three chromaticity coordinates is plotted against any other is called a chromaticity diagram. In this diagram, the chromaticity of a color stimulus plots as a point, the chromaticity point.
Generally, chromaticity coordinates are derived from the color matching functions. The CIE 1931 2-deg CMFs (CIE, 1932), however, were derived from the chromaticity coordinates of Wright (1928) and Guild (1931).
CIE (1932). Commission Internationale de l'Éclairage Proceedings, 1931. Cambridge: Cambridge University Press.
Guild, J. (1931). The colorimetric properties of the spectrum. Philosophical Transactions of the Royal Society of London, A230, 149-187.
Wright, W. D. (1928). A re-determination of the trichromatic coefficients of the spectral colours. Transactions of the Optical Society, 30, 141-164.