A rigorous graphical method used in the analysis of the separation of two heterogeneous liquids by distillation. It is used to determine the number of stages required to bring about a required separation and is based on a stage-wise approach using enthalpy and composition of each theoretical tray or equilibrium stage. The graphical technique is based on the vapour–liquid equilibrium data for the more volatile component in the feed. The method was developed by M. Ponchon and P. Savarit independently between 1921 and 1922.