text: Despite the experimental support mentioned above, Clars rule suffers from some limitations. In the first place, Clars rule is formulated only for species with hexagonal rings, and thus it cannot be applied to species having rings different from the benzene moiety, even though an extension of the rule to molecules with rings of any dimension has been provided by Glidewell and Lloyd. Secondly, if more than one Clar structure exist for a given species, Clars rule does not allow to determine the relative importance of each of them in the determination of the physicochemical properties. Finally, it is important to mention that exceptions to the Clars rule exist, such as in the case of  triangulenes.