Abstract Symmetry requirements are not the only principles that govern the electron structure of open shell states. The hole-electron equivalency also can act as a genuine building-up rule, that determines formal characteristics of the resulting states. These ideas are used to provide an insight in the nature of diradical states in organic chemistry. A group theoretical derivation of the possible coupling schemes for an e 2-electron system (twisted ethylene, cylobutadiene, trimethylenemethane, etc…) is presented. The chemical reactivity patterns of these species are related to the building-up principles they incorporate.