As the term itself implies, polytonality is the incorporation of two or more tonalities in the music at the same time. Polytonality often uses two or more highly disjunctive key areas so that there is a distinct difference. This technique is effective when music played by the instrument or group of instruments is diatonic to the key area. When the key areas become chromatic the effect of polytonality becomes blurred and to some extent indefinable.
Charles Ives experimented with polytonality in some of his orchestral works. Sections of his Symphony No. 4 explore different key areas at the same time. Igor Stravinsky’s Petroushka juxtaposes the key areas of f-sharp major and c major in the famous Petroushka motive. Bela Bartók incorporated polytonality in many of his chamber works. Some simple, early examples can be found in his Mikrokosmos for solo piano. Bartók used this technique often in his 44 duos for two violins, as a way to intertwine two folk melodies of contrasting harmonic centers with similar tempo and mood.
Definition provided by Brian Bice