A result that states that if the potential energy curves for the electronic states of a diatomic molecule are plotted as functions of the internuclear distance, then curves for two electronic states with the same symmetry cannot cross each other. This rule is very useful in the construction of correlation diagrams and in the electronic spectroscopy of diatomic molecules. The non-crossing rule has to be considerably modified in the case of polyatomic molecules, it being possible for the potential energy surfaces of two electronic states with the same symmetry to cross. The circumstances under which such crossings for polyatomic molecules can occur have been extensively investigated.