  (3) KS illustrate orthogonality relations of rays by graphs which have come to be called KS diagrams. In such a diagram each ray (or point specifying a ray) is represented by a vertex. Vertices joined by a straight line represent orthogonal rays. The colouring problem then translates into the problem of colouring the vertices of the diagram white or black such that joined vertices cannot be both white and triangles have exactly one white vertex.