On the recently recorded-high resolution STM images of large polycyclic aromatic hydrocarbons (PAHs), submolecularly resolved patterns are observed, that differentiate between parts of the molecules in which, according to the Clar aromatic sextet theory, the 'full' and the 'empty' rings are located. This phenomenon has been explained by the shape of the frontier orbitals of the PAHs, usually by the HOMO or LUMO. We now put forward a simple theoretical model of the distribution of pi-electrons in PAHs, that results in patterns remarkably similar to those observed in the STM images. A possible explanation for this coincidence is offered. (C) 2004 Elsevier B.V. All rights reserved.