A rule used to discuss the aromaticity of polycyclic aromatic hydrocarbons (see aromatic compounds). It states that the resonance structure with the highest number of disjoint aromatic pi-sextets has the biggest influence on the aromatic nature of the molecule. A notable example of Clar’s rule is that in phenanthrene one resonance structure has two disjoint sextets and the other has one, whereas in anthracene there can be one sextet in each of the three rings, thus explaining why phenanthrene is more stable than anthracene. The rule was stated by Erich Clar (1902–87) in 1964.