Thus it was noted that along the reaction coordinate of pericyclic processes one could have either a Möbius or a Hückel array of basis orbitals. With 4n or 4n + 2 electrons, one is then led to a prediction of allowedness or forbiddenness. Additionally, the M–H mnemonics give the MOs at part reaction. At each degeneracy there is a crossing of MOs. Thus one can determine if the highest occupied MO becomes antibonding with a forbidden reaction resulting. Finally, the M–H parity of sign inversions was utilized in the 1970 W–H treatment of allowedness and forbiddenness. The parity of sign inversions between bonds and atoms was used in place of the M–H use of atoms; the two approaches are equivalent.