In general, the chemical structure of a given polycyclic aromatic hydrocarbon admits more than one  resonance structure: these are sometimes referred to as Kekulé resonance structures. Some of such structures may contain aromatic π-sextets, namely groups of six π-electrons localized in a benzene-like moiety and separated by adjacent rings by  formal C–C bonds. An aromatic π-sextet can be represented by a circle, as in the case of the anthracene molecule. Clars rule states that for a benzenoid polycyclic aromatic hydrocarbon (i.e. with only hexagonal rings), the resonance structure with the largest number of disjoint aromatic π-sextets is the most important to characterize its chemical and physical properties. Such resonance structure is called the Clar structure. In other words, a polycyclic aromatic hydrocarbon with a given number of π-sextets is more stable than its isomers with less π-sextets. In 1984, Glidewell and Lloyd provided an extension of Clars rule to polycyclic aromatic hydrocarbons containing rings of any size. More recently, Clar's rule was further extended to biradicaloids in their singlet state.