The main reason for sowing the example above is to demonstrate that mass and component balance are easily represented by lines on the equilibrium diagram, and this allows the balance to be solved simultaneously with the necessary equilibrium condition. We will use this representation of the balance throughout this course, and in this chapter, particularly in the section on the McCabe-thiele method of distillation column design. In these designs we will have a problem involving two feeds and two products, with neither the feeds nor the products necessarily being in equilibrium: this does not affect the accuracy of the line representing the balances.