  Consider the hue circle in figure 2, taken from the Natural Color System color space, which is organized around the four hues red, green, blue, and  yellow.[8]  These are the unique hues: they have shades that are not perceptual mixtures of any other hue. For example, there is a shade of green that is neither bluish nor yellowish. The other hues are binary: all of their shades are perceptual mixtures of two unique hues. For example, every shade of purple is bluish and reddish. The four unique hues—yellow, red, blue, green—come in two opponent pairs: red-green and blue-yellow. Red is opposed to green in the sense that there are no reddish-greens or greenish-reds; likewise there are no bluish-yellows or yellowish-blues. The unique hues are equally spaced around the NCS circle, and the other hues are spaced according to their perceptible proportion of the unique hues. So, for example, the orange patch equidistant from the unique yellow pole and unique red pole is equally similar to both.