Here for the first time is an orderly presentation of the harmonic procedures to be found in music of the first half of the twentieth-century. The author examines the nature of intervals in various contexts, discusses the modes and other scales employed in modern music, describes the formation and uses of chords by thirds, by fourths, and by seconds, of added-note chords and polychords; he deals with different types of harmonic motion, with harmonic rhythm and dynamic sand ornamentation, with harmonic behavior in tonality, polytonality, atonality and serial composition.