A set of rules relating the structures of certain alloys to the electron concentration, i.e. the number of valence electrons per atom in the structure. For example, b.c.c. structures are associated with an electron concentration of 3/2. The rules were proposed by the British metallurgist William Hume-Rothery (1899–1968) in the 1920s and 1930s. In 1934 an explanation was suggested in terms of the Fermi surface touching the planes of the first Brillouin zone of alloys. However, in the 1950s experimental investigation of the Fermi surface showed that this explanation was too simple. In the 1990s explanations were proposed in terms of realistic descriptions of energy bands and also in terms of electron counting rules. Later work showed how to combine the energy band and electron counting approaches.