Chapter 2 explains T–cphase diagrams, which are maps of equilibrium alloy phases in a space spanned by temperature T and chemical composition c. The emphasis is on deriving T–c phase diagrams by minimizing the total free energy of an alloy with two or three phases. The lever rule and common tangent constructions are developed. Some basic ideas about chemical interactions and entropy are used to justify of the free energies of alloy phases at different temperatures. For binary alloys, the shapes of free energy versus composition curves and their dependence on temperature are used to deduce eutectic, peritectic, and continuous solid solubility phase diagrams. Some features of ternary alloy phase diagrams are discussed. If atoms are confined to sites on an Ising lattice, free energy functions can be calculated with a minimum set of assumptions about the energies of different atomic configurations. These generalizations of chemical interactions are useful for identifying phenomena common to unmixing and ordering transitions, but warnings about their limitations are presented.