In their seminal paper from 1965, where they also coined the term "electrocyclic reaction," Robert Woodward and Roald Hoffmann connected the observed behavior to the concept of the conservation of orbital symmetry . This concept can be visualized with the example of CHD (see Fig. ). During the ring-opening reaction of CHD, two π-bonds and one σ-bond are effectively transformed into a set of three π-bonds, where the positions of all π-bonds change. The reactant molecular orbitals involved in the reaction are, therefore, one σ orbital, two π orbitals, and their corresponding unoccupied π * and σ * orbitals (see Fig. , center). The corresponding product ORCID: Content not peer-reviewed by ChemRxiv. License: CC BY 4. Liu, Xu, Sanchez, Martínez, Wolf -Page 4 orbitals are three π and three π * orbitals (see Fig. , left and right). The conservation of orbital symmetry dictates that reactant and product orbitals can only correlate if their transformation with respect to the symmetry element of the molecule, which is preserved during the reaction, is the same.