The common conditions of extreme parameters of state in multicomponent multiphase systems are considered. The consideration is based on the common differential Van der Waals equations in the metric of complete and incomplete Gibbs potentials. Two fundamentally different cases of the ratio between the system state parameters, when temperature (at constant pressure), pressure (at constant temperature), components or compounds chemical potentials (at constant temperature and pressure simultaneously), phase composition parameters (also at constant temperature and pressure simultaneously) pass through the extremums are determined. Both common rules, concerning extreme state parameters are supported by the examples from the different types diagrams of phase equilibrium: liquid-vapor, liquid-solid (diagrams of solubility and fusibility).