Building on early-century intuitions and inclusive views about polytonality/polymodality as modulation in simultaneity and layered dissonance by Milhaud (1923), Koechlin (1924), Casella (1924) and Bártok (1943), the article proposes the notion of scalar dissonance as an analytical model for multilayered scale interactions. The model develops a conceptual framework and graphic representation for the combined musical structure whose workings draw on the analogy to metric dissonance theory. The framework set up stands as a regulatory principle for the interpretation of scale relations. Scalar dissonance proposes a measuring tool for the degree of mismatch or friction as well as the degree of porosity/permeability between distinct scale-segments. The analytical approach is set apart from other scale theory measuring tools such as voice leading. It grounds a listening strategy for reorientation across layers and forms the basis for an experiential approach to contrapuntal textures of polytonality, examining aspects of layer interaction and syntax in music by Milhaud, Bartók, and Ravel.