Summary

Chapter XVIII positions Messiaen's modes of limited transpositions within the landscape of early twentieth-century compositional practice by clarifying what they are not. The modes differ fundamentally from traditional modal systems (Indian, Chinese, Greek, plainchant) through their basis in symmetrical structure rather than scalar tradition. They differ from atonality by maintaining capacity to suggest tonal centers while offering distinctive harmonic colors. They differ from polytonality through a subtle but crucial distinction: rather than juxtaposing incompatible tonal centers, the modes create "the atmosphere of several tonalities at once, without polytonality"—the modal force absorbs potential polytonal implications into unified harmonic fields. Finally, Messiaen acknowledges that analogous systems of limited-transposition modes could exist in quarter-tone systems but declines to explore this territory, focusing instead on the complete set of seven modes available within twelve-tone equal temperament. This brief but conceptually dense chapter establishes Messiaen's modal practice as distinctive from other contemporary approaches while demonstrating its compatibility with various harmonic systems through judicious mixing and combination.