"Procedure for Dewar-Zimmerman analysis" (examples shown on the right): "Step 1." Shade in all basis orbitals that are part of the pericyclic system. The shading can be arbitrary. In particular the shading does not need to reflect the phasing of the polyene MOs; each basis orbital simply need to have two oppositely phased lobes in the case of p or sp"x" hybrid orbitals, or a single phase in the case of an s orbital. "Step 2." Draw connections between the lobes of basis orbitals that are geometrically well-disposed to interact at the transition state. The connections to be made depend on the transition state topology. (For example, in the figure, different connections are shown in the cases of con- and disrotatory electrocyclization.) "Step 3." Count the number of connections that occur between lobes of opposite shading: each of these connections constitutes a phase inversion. If the number of phase inversions is even, the transition state is Hückel, while if the number of phase inversions is odd, the transition state is Möbius. "Step 4." Conclude that the pericyclic reaction is allowed if the electron count is 4"n" + 2 and the transition state is Hückel, or if the electron count is 4"n" and the transition state is Möbius; otherwise, conclude that the pericyclic reaction is forbidden.