The relation is shown between the two-dimensional chromaticity diagram, and the three-dimensional region of realizable colors in tristimulus space. Among the figures included is an artistic rendering of the boundary of this region in CIE coordinates. The CIE chromaticity diagram, much used in color photographic technology, is a planar system derived from three-dimensional coordinates. As part of a recent talk, the author had occasion to explain the way in which the diagram is formed. For this purpose, a series of illustrations was produced. Since it is more explicit and complete than what we have encountered in the literature,1 we present it here with some explanatory material. To the best of our knowledge, the inclusion of a view of the three-dimensional spectrum locus is unique. The chromaticity diagram is shown in Figure 1. as it is ordinarily used. Figure 2 is a perspective view in a coordinate plane of the overall tristimulus space. For the figures which follow, it is more convenient to view toward the origin from a point in the all-positive octant. The spectrum locus is shown from this view, in Figure 3. Chromaticity coordinates X, Y and Z satisfy the relation: X+Y+Z=1. This equation holds (only) for points in the "unit" plane, which intersects the coordinate axes as shown in Figure 4. The chromaticity diagram can be regarded as the projection vertically downward, from a corresponding region in that unit plane, as shown in Figure 5.