It is possible to express the transition temperature nematic-isotropic (TNI) for nematogens with just two aromatic ring systems as the sum of incremental values of substructural elements, denoted as substituents and bridging moieties. Of the 1426 mesogens that can be constructed by the elements for which the increments have been given, the TNI values of most can be predicted to within 10 C, the only notable exception being the 1,2-diphenylethenes.