Distillation lines that are commonly used to predict feasible product regions are examined, focusing on the theoretical investigation and on the experimental verification of different models, Distillation lines based on the equilibrium stage model, residue curves, fluctuation transport theory, and multicomponent diffusion theory are derived and compared with experiments. The comparison with experiments shows that the consideration of the diffusional interactions between the components may be of essential importance when the column is operated close to a distillation boundary.