This lecture covers the principles behind the Mollier diagram, used for calculating thermodynamic properties in engineering projects, with a focus on specific enthalpy and entropy. It explains the isobaric lines, the liquid zone, the dry vapor zone, and the critical point. Additionally, it delves into the Log p-h diagram, commonly used in refrigeration systems, detailing the liquid, vapor, and mixture zones. The lecture concludes by highlighting the practical applications of these diagrams in engineering projects.