In  organic and physical organic chemistry, Clars rule is an  empirical rule that relates the chemical stability of a molecule with its aromaticity. It was introduced in 1972 by the  Austrian organic chemist Erich Clar in his book The Aromatic Sextet'. The rule states that given a polycyclic aromatic hydrocarbon, the resonance structure most important to characterize its properties is that with the largest number of aromatic π-sextets i.e. benzene-like moieties.