A set of semi-empirical rules were developed by John B. Goodenough and Junjiro Kanamori in the 1950s. These rules, now referred to as the Goodenough–Kanamori rules, have proven highly successful in rationalizing the magnetic properties of a wide range of materials on a qualitative level. They are based on the symmetry relations and electron occupancy of the overlapping atomic orbitals (assuming the localized Heitler–London, or valence-bond, model is more representative of the chemical bonding than is the delocalized, or Hund–Mulliken–Bloch, model). Essentially, the Pauli exclusion principle dictates that between two magnetic ions with half-occupied orbitals, which couple through an intermediary non-magnetic ion (e.g. O), the superexchange will be strongly anti-ferromagnetic while the coupling between an ion with a filled orbital and one with a half-filled orbital will be ferromagnetic. The coupling between an ion with either a half-filled or filled orbital and one with a vacant orbital can be either antiferromagnetic or ferromagnetic, but generally favors ferromagnetic. When multiple types of interactions are present simultaneously, the antiferromagnetic one is generally dominant, since it is independent of the intra-atomic exchange term. For simple cases, the Goodenough–Kanamori rules readily allow the prediction of the net magnetic exchange expected for the coupling between ions. Complications begin to arise in various situations: