CIE Chromaticity Diagram
The horseshoe-shaped plot of all colours visible to human vision, on which colour space gamuts are drawn as triangles.
Measured in
CIE xy
In depth
The curved boundary traces the pure spectral colours from violet to red, closed by a straight line of purples that exist only as mixtures. Because a display's three primaries define a triangle inside this shape, the diagram makes immediately clear how much of visible colour any space actually covers — and why no three-primary system can cover it all.
Examples in practice
Plotting sRGB and ProPhoto on the diagram shows the latter's triangle extending beyond the visible horseshoe entirely.