The CIE's chromaticity diagram, developed in 1931, is still used as a standard reference for defining colours, and as a reference for other color spaces. The diagram is a two-dimensional plot of colours of constant intensity based on the visual response of the CIE-1931 standard observer, which was determined by physiological measurements of human colour vision. Since the human eye has three types of colour sensor that respond to different ranges of wavelengths, a full plot of all visible colours is a three-dimensional figure. This is inconvenient to draw on a two-dimensional sheet of paper, so for convenience the CIE transformed the three-dimensional colour space into two artificial dimensions of colour (collectively called chromaticity) and one of intensity, and then took a two-dimensional slice through this space at the level of maximum intensity. This slice became the chromaticity diagram. Incidentally, this technique of converting a three-dimensional colour space to a combination of chromaticity and intensity is also used in colour television.
The gamut of all visible colours on the CIE plot is a tongue-shaped or horseshoe-shaped figure, with the curved edge corresponding to the colours of the visible spectrum and the straight edge (the purple line) corresponding to non-spectral shades of purple. Less saturated colours appear in the interior of the figure, with white at the centre.
A three-dimensional figure can be made by plotting the CIE's chromaticity on two axes and intensity on the third axis. It is a roughly pyramidal solid that is informally called the colour bag.