Rules governing the formation of products during certain types of concerted reactions. The theory of such reactions was put forward in 1965 by Robert Woodward and Roald Hoffmann (1937– ), and is concerned with the way that orbitals of the reactants change continuously into orbitals of the products during reaction and with conservation of orbital symmetry during this process. It is closely related to frontier-orbital theory. These rules have mostly been applied to organic reactions. The Woodward–Hoffmann rules were originally derived using molecular-orbital theory but can also be derived using valence-bond theory.