In recent research, it has been increasingly necessary to employ an extended wavelength metric to cover the complete hue cycle so as to research or represent data as function of relative wavelength rather than psychological scales such as CIELAB or Munsell hue angle. This article describes such a relative wavelength metric and its derivation from complementary wavelength functions. The metric provides a useful psychophysical, wavelength-based, ratio scale for the hue cycle allowing nonspectrals to be treated fit the same coherent scale as spectral hues. Several indicators, e.g., color order hue cycles. infer the (optimally efficient) spectral hues comprise about 71% and the nonspectrals about 29%, of the hue cycle interval. This gives a hue cycle whose relative wavelength interval is about 240 nm for illuminant D65. To relate to CIE colorimetry, spectral complementary wavelengths to the nonspectrals relative wavelenghts are identified for seven illuminants including D65, D50, C, and A. Data are given for CIE 1931. and briefly 1964, colorimetry. (C) 2010 Wiley Periodicals, Inc. Col Res Appl. 35, 122-133 2010: Published online 7 January 2010 in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/col.20534