The 8-N rule by Hume-Rothery (1931) for structures of main group elements relates the number of covalent atom bonds with the group number, i.e. the number of valence electrons. The atoms, by forming shared two-electron bonds, complete their octets. Hume-Rothery's idea was subsequently applied to rationalize anion-anion bonds in ionic compounds. A generalized 8-N rule which considers both anion-anion and cation-cation bonds was published in 1964/65. It can be used with iono-covalent and also strongly polar intermetallic compounds, the latter referred to as Zintl phases. Structural features are interpreted in terms of an underlying ionic bonding scheme where electropositive elements transfer valence electrons to electronegative elements which complete their octets, if necessary by forming homonuclear bonds. The generalized 8-N rule has recently been combined with other valence rules to derive for ionocovalent compounds possible base polyhedra, small building units, with which can be constructed neutral and anionic polyhedron complexes.