AbstractDominant wavelength is a psychophysical scale presently limited to the spectrum. It is extended into the nonspectrals to designate the hue cycle in a continuous wavelength‐based scale. To omit the fading spectrum ends, the extended scale is based on the limited spectrum of optimum color stimuli (442–613 nm). The hue cycle interval is found by many methods in agreement, e.g., from the spacing of the ideal primaries (additive and subtractive). These were recently defined as the complementary maxima and minima of several visual functions (e.g., saturation/W, spectral sensitivity, λ discrimination, complementary efficiency). Five of these six primaries are spectral, uniformly spaced at (40 ± 4) nm intervals. Interpolating the sixth (531 c magenta) as similarly spaced between adjacent primaries (blue 447 nm and red 607 nm) gives it the dual designation 407/647 as both ends of the cycle, an interval equivalent to 240 nm. Nonspectral hues are numbered serially with wavelength by interpolating green–purple complementary pairs as sinusoidallike spectral pairs. Applying the extended scale to a color circle, the six primaries reveal a sextuple symmetry of (60 ± 2)°. Only these three pairs are both complements and opposites. In a graph (hue cycles as x and y axes) of x + y color‐mixture, loci of constant λ center symmetrically on the RGB primaries near 607, 531, 447 nm. © 1993 John Wi