This theory was originally introduced to permit qualitative study of the π-electron systems in planar,
conjugated hydrocarbon molecules (i.e. in "flat" hydrocarbon molecules which possess a mirror
plane of symmetry containing all the carbon atoms, and in which the atoms of the carbon skeleton
are linked by alternating double and single carbon-carbon bonds when the bonding is represented in a
localised fashion). It is thus most appropriate for molecules such as benzene or butadiene, but the
approach and concepts have wider applicability.