Charles Ives used not only polytonality in his chamber works, but also polymeter.
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Williams combined polytonality and the Lydian mode to express a mystic, dreamlike and heroic quality.
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His technique included the use of polytonality, polyrhythm, tone clusters, aleatoric elements, and quarter tones.
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Polytonality figures prominently in the major operas as a mechanism for manifesting the interaction between characters.
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For the musical term, see polytonality.
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Hints of jazz or riffs on Beethoven often peek through dense canvases rich in polytonality, rhythmic complexity and thematic layering.
From the nytimes.com
Darius Milhaud developed the use of polytonality, that is, music where different instruments play in different keys at the same time.
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Rather than simply set the melodies in an orchestral setting, Mosolov used dense textures and polytonality that disregarded the style of socialist realism.
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Williams took a modernist approach, especially with his use of polytonality, which refers to the sound of two different keys played simultaneously.
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More examples
Music that uses two or more different keys at the same time
(polytonal) using more than one key or tonality simultaneously; "exciting rhythms and polytonal harmonies"
The use of two different keys simultaneously. Despite much loose talk, true polytonality is rare. Upper structures (q.v.) and outside playing do not usually qualify because there is always a strong single underlying tonality.
The use of two or more keys simultaneously. Employed by Stravinsky, Holst and Milhaud. When only two keys are used, this is referred to as bitonality.