Interfacial energy gamma(1,2) of a liquid-liquid or solid-liquid system is of paramount importance in colloid and interface science and its applications. To assess the dependence of gamma(1,2) on the surface energies gamma(1) and gamma(2) of two materials in contact, Girifalco and Good proposed their venerable equation involving the interfacial interaction parameter phi Subsequently, values of phi have been experimentally determined for various material pairs. Here, we show that, the data of phi closely follow a unique relationship phi = (1 - gamma(1,2)/gamma(1))(1/2) for all pairs where the other material is non-polar. Theoretically, this curve describes the smallest possible phi However, we also show that substituting this relationship into the Girifalco-Good equation reduces it to Antonov's rule gamma(1,2) = gamma(1)-gamma(2). Such a simplistic approach is inaccurate, and we conclude that the plotting of phi vs. gamma(1,2) has contributed to overestimating the applicability of the Girifalco-Good Equation. (C) 2018 The Authors. Published by Elsevier Inc.