text: Since its formal statement in 1972, Clars rule has received a vast amount of experimental evidence. The dependence of the  color and reactivity of some small polycyclic aromatic hydrocarbons on the number of π-sextets in their structures is reported by Clar himself in his seminal contribution. Similarly, it was shown that the  HOMO-LUMO gap, and therefore the color, of a series of heptacatafusenes depends on the number of π-sextets. Clars rule has also been supported by experimental results about the distribution of π-electrons in polycyclic aromatic hydrocarbons, valence bond calculations, and nucleus independent chemical shift studies.
  Clars rule is widely applied in the fields of chemistry and materials science. For instance, Clars rule can be used to predict several properties of graphene nanoribbons. Aromatic π-sextets play an important part in the determination of the ground state of open shell biradical-type structures. , Clar's rule can rationalize the observed a decrease of the bandgap of holey graphenes with increasing size.