The generalized Woodward–Hoffmann rules, first given in 1969, are equivalent to an earlier general approach, the "Möbius-Hückel concept" of Zimmerman, which was first stated in 1966 and is also known as "aromatic transition state theory". As its central tenet, aromatic transition state theory holds that 'allowed' pericyclic reactions proceed via transition states with aromatic character, while 'forbidden' pericyclic reactions would encounter transition states that are antiaromatic in nature. In the Dewar-Zimmerman analysis, one is concerned with the topology of the transition state of the pericyclic reaction. If the transition state involves 4"n" electrons, the Möbius topology is aromatic and the Hückel topology is antiaromatic, while if the transition state involves 4"n" + 2 electrons, the Hückel topology is aromatic and the Möbius topology is antiaromatic. The parity of the number of "phase inversions" (described in detail below) in the transition state determines its topology. A Möbius topology involves an "odd" number of phase inversions whereas a Hückel topology involves an "even" number of phase inversions.