The bond-valence sum rule has been examined by molecular-orbital methods related to spin-coupling matrix theory [Okada & Fueno (1976). Bull. Chem. Soc. Jpn, 49, 1524-1530], to give a new formulation of the Lewis-electron pair concept. It is shown that the 'pair-coupling population' between atoms M and X exhibits the same behaviour as the bond valence between them. A quantum chemical definition for bond valence is proposed and successfully applied to Al(2)Cl(6), Te(4)Cl(16) and Al(2)Be(3)(SiO(3))(6) (beryl). Using an alternative bond-valence definition it is shown that for oxides the bond valence can possibly be taken as the double pair-coupling population.