This document discusses binary phase diagrams and the principles that govern them. It introduces the concepts of complete and limited solubility in liquid and solid states, which determine the different types of binary phase diagrams. It also discusses the phase rule and its application to binary systems. Additionally, it explains the theorem of Le Châtelier, Clausius-Clapeyron equation, lever rule, and how to determine the composition and relative amounts of phases that coexist at a given temperature based on a binary phase diagram. Examples are provided to demonstrate determining phase fractions using the lever rule.