Summary

Chapter XV reconceives traditional nonharmonic tone categories by enlarging them from individual pitches into complete musical structures—pedal groups, passing groups, and embellishment groups—each possessing internal rhythmic, harmonic, and melodic organization. The chapter's central contribution is the upbeat-accent-termination complex, which Messiaen identifies as the most expressive combination of foreign notes in both classical and modern styles. By allowing these elements to expand to "immense" proportions and to function discontinuously (separated by rests or appearing without expected complements), Messiaen provides a theoretical framework for understanding gestural articulation and phrase structure in twentieth-century music. The integration of these harmonic concepts with rhythmic procedures and modal techniques demonstrates the synthetic nature of Messiaen's compositional thinking, where all musical parameters collaborate toward unified expressive goals.