Gibbs' Phase Rule

Gibbs' phase rule states that the number of degrees of freedom in a system at equilibrium equals the number of independent components minus the number of coexisting phases, plus two, for temperature and pressure: F equals C minus P plus 2. Josiah Willard Gibbs derived it in his 1876 to 1878 papers on the equilibrium of heterogeneous substances, and it is the rule that tells a phase diagram how many phases can coexist at a given composition and temperature, making it foundational to how materials science reads and constructs phase diagrams for alloys, ceramics and other multicomponent materials.