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Bimodality is the practice of using notes from two distinct pitch classes in a single musical composition, making the key or tonal center more ambiguous. It thus is a technique designed to provide more interesting harmonization and push the boundaries of tonal music. It is considered a more contemporary method and is closely related to the musical concept of polytonality.
Bimodality can use any two pitch classes of the composer's choosing. The classes do not have to be what modern composers and listeners know as "major" or "minor" scales. They also can use the modes developed by the ancient Greeks, which include the Dorian, Phrygian, Lydian, Mixolydian, Aeolian, Locrian, and Ionian modes. Other possible classes include whole tone and pentatonic scales.
The primary difference between bimodality and a simple change in key or mode is that bimodality requires the two modes to be present simultaneously. For example, the notes of the Lydian mode starting on F are F, A, B, C, D and E. The notes of the Dorian mode starting on D are D, E, F, G, A, B and C. If one player played in Lydian for measures one through four and a second player performed in Locrian for measures one through four, the piece would be truly bimodal. If the work had everyone play in Lydian in measures one through four and everyone play in Locrian in measures five through eight, however, the piece would not fit the bimodal definition.
Normally, in a work with only one mode or tonal center, all lines work together within the same harmonic progression, even though each line can be rhythmically and melodically independent. In bimodality, this is no longer true. The harmonies present may fit either pitch class. Often this results in a high level of dissonance, or at the very least, creates more complex chords.
For a composer who does not want as much clashing between pitches, the challenge of bimodality is to find the commonalities and relationships between the pitch classes and not deviate from them. For instance, using the Lydian on F and Dorian on D example, a composer might notice that a D minor chord with notes D, F and A is possible in both modes. In Lydian, the D minor chord would be built on the fifth note of the mode. In Dorian, it would be built on the first note of the mode. The composer might also notice that the distance between the first two notes of the two modes is a third, which forms a mediant relationship.
Bimodality should not be confused with mode mixing. In mode mixing, composers simply borrow harmonies freely between a major key and its relative minor. This gives the composer a greater ability to add more color to the work and use different types of progressions and chord relationships, but the modes alternate instead of having both present at the same time. The fact the composer cannot use melodies or harmonies from both the major and minor keys simultaneously differentiates this technique from bimodality.