An electron concentration parameter, expressed as the number of itinerant electrons per number of atoms N in a structural unit cell of an alloy, e/uc, is a useful parameter that can be used in interpreting the origin of a band structure pseudogap often evident at the Fermi level for structurally complex metallic alloy phases. It can be expressed in terms of the interference condition e/uc =3/2 , where |G| 2 c is the square of the critical reciprocal lattice vector associated with the specic set of lattice planes interfering with electrons at the Fermi level. This parameter is similar to the well-known Hume-Rothery electron concentration parameter e/a, which represents the number of electrons per atom and is linked with e/uc through the relation e/uc = N (e/a). We have demonstrated that certain complex metallic alloy structures appear to be stable at or near certain values of e/a. We show that the e/a = 1.60 rule holds for the sub-group of gamma-brasses with space group I43m and Pearson symbol cI52, the e/a = 4.34 rule for skutterudite compounds with Im3 and cI32, the e/a = 2.74 rule for Al6TM (TM = Mn, Tc, Re, Fe, and Ru) compounds with Cmcm and oC28, the e/a = 1.62 rule for the sub-group of the Heusler compounds with F m3m and cF 16, and the e/a = 2.09 rule for the sub-group of Zintl compounds MX1 (M = Li and Na, X1 = Al, Ga, In, and Tl) with F d3m and cF 32. The e/a rule holds in sub-groups of isostructural compounds, regardless of the degree of orbital hybridizations and the polarity involved.