Recent studies of the pi-electron properties of acenaphthylene, fluoranthene, and their congeners showed that the presence of phenyl-cyclopentadienyl (PCP) fragments amplifies the intensity of cyclic conjugation in the five-membered ring of the respective molecules. We now show that there exists a reasonably good correlation between two quantum mechanical measures of cyclic conjugation in the five-membered rings (the energy effect and the pi-electron energy content) and the number of six-membered rings in PCP-constellations. We also offer a mathematical explanation of the PCP effect. (c) 2009 Elsevier B.V. All rights reserved.