To determine symmetry and asymmetry with respect to σ1 and σ2, the starting material molecular orbitals must be considered in tandem. The figure to the right shows the molecular orbital correlation diagram for the [π2s + π2s] cycloaddition. The two π and π* molecular orbitals of the starting materials are characterized by their symmetry with respect to first σ1 and then σ2. Similarly, the σ and σ* molecular orbitals of the product are characterized by their symmetry. In the correlation diagram, molecular orbitals transformations over the course of the reaction must conserve the symmetry of the molecular orbitals. Thus πSS correlates with σSS, πAS correlates with σ*AS, π*SA correlates with σSA, and finally π*AA correlates with σ*AA. Due to conservation of orbital symmetry, the bonding orbital πAS is forced to correlate with the antibonding orbital σ*AS. Thus a high barrier is predicted.