P-x diagram (vapour pressure vs mole fraction): shows the liquid line (Raoult's law) and vapour line. For ideal solutions, both are smooth curves between P_A_{standard} and P_B_{standard}. The region between them is the two-phase (liquid + vapour) region. T-x diagram (boiling point vs composition): inverted relationship — lower vapour pressure means higher boiling point. The liquid curve (lower) and vapour curve (upper) define the two-phase region. For positive deviation: P-x curve has a maximum (azeotrope point). T-x curve has a minimum. For negative deviation: P-x curve has a minimum, T-x curve has a maximum. Lever rule: in the two-phase region, the ratio of liquid to vapour is given by the lengths of tie-line segments.