A rule predicting that in covalent solids made up of two elements with an average of four outer electrons per atom, both types of atom have tetrahedral coordination in the solid. It was suggested by the German chemist Hans Georg Grimm (1887–1958) and Arnold Sommerfeld in 1926. Examples of the Grimm–Sommerfeld rule are silicon carbide, gallium arsenide, and cadmium sulphide.