Summary

Chapter XIX presents polymodality—the simultaneous superposition of multiple modes of limited transpositions—as the culminating technique of Messiaen's harmonic practice. Beginning with two-mode combinations (the fundamental polymodal texture), advancing through three-mode superpositions (creating maximum harmonic density), and concluding with polymodal modulation (large-scale transformation of modal combinations), the chapter demonstrates systematic approaches to creating complex stratified textures while maintaining the distinctive character of each constituent mode. The technique differs fundamentally from polytonality through the "modal force" that absorbs potential tonal conflicts into unified harmonic atmospheres. Throughout, polymodality integrates with rhythmic procedures—pedal groups, rhythmic canons, added values, polyrhythm—demonstrating Messiaen's holistic approach where all musical parameters operate according to parallel organizational principles. The chapter concludes the treatise's harmonic section by showing how the modes of limited transpositions can be combined and transformed at multiple hierarchical levels, providing composers with resources for creating harmonic complexity and coloristic variety while avoiding both traditional tonal functions and undifferentiated chromaticism. Polymodality thus represents the full realization of the "theological rainbow" Messiaen envisioned—a musical language based on symmetry, impossibility of transposition, and the mysterious charm of existing simultaneously in multiple tonal atmospheres without committing to any single one.