There is no simple definition of the slope of the equilibrium line multicomponent systems because vapor liquid equilibrium is represented by a surface. This paper describes a method of approximating the slope in multicomponent systems based on the new concept the design component, defined as that compound we seek to eliminate from the mixture in a particular section of the column. Unlike traditional methods, this new approach can also be applied to absorber and stripper columns. Two analytical approximate methods of calculating the design slope are described and shown to be in good agreement with the rigorous design flash calculation even in cases where the relative volatility is far from constant. These simple approximations, which need nothing beyond what is readily available from any column simulation program, appear to be adequate for design and can be easily programmed in any spreadsheet.