The detailed analysis of the well-known relation gamma(12) = gamma(1) - gamma(2) has been done (gamma(12), gamma(1) and gamma(2) are interfacial and surface tensions of phases 1 and 2). The analysis of equilibrium of a liquid drop at the interface with another liquid allowed us to prove that this relation should be modified by including in it the value gamma(2)(*) which is considered as the result of all interactions at the interface and is accepted depending on gamma(1). The value gamma(2)(*) represents the surface tension of a phase in the ternary point solid-liquid-gas, i.e. in the zone of interfacial nonuniformity. The modified form of the relation, called the rule of interfacial equilibrium, allows us to show that thermodynamic work of adhesion is equal to the cohesion energy of the interphase formed by phase 2.