This document discusses thermal equilibrium diagrams, also known as phase diagrams. It defines key terms like phase, system, components, and equilibrium. It explains that phase diagrams show the phases that exist at equilibrium for different combinations of temperature and composition. They are useful for studying phase separation, solidification, heat treatment effects, and other structural changes. However, they only apply under equilibrium conditions, which are difficult to achieve in practical applications like welding. The document also classifies phase diagrams as unary, binary, ternary, etc. based on the number of components and describes common features like liquidus lines, solidus lines, and how to determine phase compositions.