The principles concerning the evolution of pinch-point zones with variation of the reflux ratio in the rectifying section of a multicomponent distillation column with an infinite number of stages are presented in this work. Analytical formulas are obtained for calculating the pinch-point zones as well as the distillate exactly. It has been found that the evolution of pinch-point zones in the stripping section is much similar to the evolution of pinch-point zones in the rectifying section. Based on these new findings, a new method for calculating the minimum reflux ratio of multicomponent continuous distillation, as well as a new shortcut method for the design calculation of a batch distillation process, can be developed.