The interactions between reaction and separation in extractive and reactive cascades are addressed through the use of difference points. It is shown how the structural elements in a generic cascade such as (side) feeds, products and chemical reactions combine to a normalized linear combination where the effect of each individual contribution is readily visualized. We derive and analyze the fundamental mathematical and geometric properties of these difference points for individual and aggregated units. Finally, it will be shown how to draw straight sectional and overall material balance lines in composition space: for reacting systems through the use of pseudo-compositions.