Audio: polytonality (0:08)
polytonality plays polychords on a brass synth. The polytonality figure shows the score:
Polytonality is the simultaneous use of multiple keys.
Bitonality or bimodality is harmony in two simultaneous keys.
Polychord is an umbrella term for a chord formed from two or more keys.
A polychord can be treated as two distinct chords written in two different keys or as a single composite chord written in a single key.
A polychord treated as a single chord is notated as a seventh chord, extended chord or altered chord.
The chord Cmaj7, for example, the first chord in polytonality, is a seventh chord containing the notes CEGB. The chord can be split into two triads, a C major triad, CEG, and an E minor triad, EGB. These two chords form the basis for a bitonal piece in C major and E phrygian. The chord Cmaj9, in bar 2, is an extended chord containing the notes CEGBD. It is formed from two overlapping triads, C major, CEG, and G major, GBD. It is the basis for a bitonal piece in C major and G major.
A polychord treated as two distinct chords has a special form of polychord notation, shown in bar 3 of polytonality. The polychord notation displays the two chords as a fraction, with one chord on top of the other. The upper chord is the chord in the treble staff and the lower chord is the chord in the bass staff.
Some polychords are so famous they have a name instead of a number. The Petrushka polychord in bar 4 is an altered chord. It is formed from two tonic triads in the distant keys of C major and Gb major. These two keys are a tritone apart. A polychord in distant keys sounds more dissonant than one in close keys.