For each component in a binary mixture, one could make a vapor–liquid equilibrium diagram.  Such a diagram would graph liquid mole fraction on a horizontal axis and vapor mole fraction on a vertical axis.  In such VLE diagrams, liquid mole fractions for components 1 and 2 can be represented as  and  respectively, and vapor mole fractions of the corresponding components are commonly represented as  and .  Similarly for binary mixtures in these VLE diagrams: 
: and  
Such VLE diagrams are square with a diagonal line running from the () corner to the () corner for reference.
These types of VLE diagrams are used in the McCabe–Thiele method to determine the number of equilibrium stages (or theoretical plates) needed to distill a given composition binary feed mixture into one distillate fraction and one bottoms fraction.  Corrections can also be made to take into account the incomplete efficiency of each tray in a distillation column when compared to a theoretical plate.