Correlation lines are drawn to connect molecular orbitals in the starting material and the product that have the same symmetry with respect to the conserved symmetry element. In the case of the conrotatory 4 electron electrocyclic ring closure of 1,3-butadiene, the lowest molecular orbital Ψ1 is asymmetric (A) with respect to the C2 axis. So this molecular orbital is correlated with the π orbital of cyclobutene, the lowest energy orbital that is also (A) with respect to the C2 axis. Similarly, Ψ2, which is symmetric (S) with respect to the C2 axis, is correlated with σ of cyclobutene. The final two correlations are between the antisymmetric (A) molecular orbitals Ψ3 and σ*, and the symmetric (S) molecular orbitals Ψ4 and π*.