In thermodynamics, phase equilibria are equilibria between substances in different phases. Phase diagram, basic to engineering, metallurgy, and mineralogy, are empirical graphs showing what phases exist at different pressures and temperatures. The phase rule, deduced by Josiah Gibbs, states that, for a closed system, F = C – P + 2, where P is the number of phases, C the number of independent chemical components, and F the number of degrees of freedom, i.e., the number of variables (pressure, temperature, composition) whose values must be specified to define the system.