Relationship to Other Chapters

Chapter XVIII serves an important clarifying function, positioning the modes within contemporary compositional discourse. It directly references Chapter XVI's theoretical foundation while distinguishing Messiaen's approach from other twentieth-century harmonic systems. The examples of mixing modes with traditional modal music (Example 182 from Chapter XII) and atonal music (Example 310 from Chapter XV) demonstrate the flexibility of modal practice already explored in Chapter XVII. The discussion of polytonality anticipates Chapter XIX's treatment of polymodality, where the distinction between these concepts will be demonstrated through practical examples. The brief mention of quarter-tone systems connects to the mathematical completeness emphasized in Chapter XVI—the seven modes represent all possible modes of limited transpositions within twelve-tone equal temperament, just as quarter-tone systems would yield their own complete set. Throughout, the chapter reinforces the fundamental aesthetic principle from Chapter I: Messiaen's modes create a distinctive harmonic atmosphere—"the charm of impossibilities"—that differs from all other contemporary approaches while remaining compatible with them in various combinations.