Bimodality is the use of notes from two different pitch classes in a single musical composition, resulting in a more ambiguous key or tonal center. As a result, it’s a method for creating more interesting harmonization and pushing the boundaries of tonal music. It is a more modern method, and it is closely related to the musical concept of polytonality.
The composer can use any two pitch classes he or she wants for bimodality. The classes don’t have to be in the “major” or “minor” scales that modern composers and listeners are familiar with. They can also use the Dorian, Phrygian, Lydian, Mixolydian, Aeolian, Locrian, and Ionian modes, which were developed the ancient Greeks. Whole tone and pentatonic scales are two other types of scales that can be used.
The main distinction between bimodality and a simple change in key or mode is that bimodality necessitates the presence of both modes at the same time. F, A, B, C, D, and E are the notes of the Lydian mode starting on F, for example. D, E, F, G, A, B, and C are the notes of the Dorian mode, which begins on D. The piece would be truly bimodal if one player played Lydian for measures one through four and a second player played Locrian for measures one through four. The piece would not meet the bimodal definition if everyone played Lydian in measures one through four and Locrian in measures five through eight.
Even though each line can be rhythmically and melodically independent, in a work with only one mode or tonal center, all lines work together within the same harmonic progression. This is no longer the case in bimodality. The harmonies could belong to either pitch class. This often results in a lot of dissonance, or at the very least, more complicated chords.
The challenge of bimodality for a composer who does not want as much pitch clashing is to find the commonalities and relationships between the pitch classes and not deviate from them. For example, a composer might notice that a D minor chord with notes D, F, and A is possible in both Lydian on F and Dorian on D modes. The D minor chord would be built on the mode’s fifth note in Lydian. It would be built on the first note of the mode in Dorian. The composer may also notice that the distance between the first two notes of the two modes is a third of the distance between the first two notes of the two modes, forming a mediant relationship.
Mode mixing should not be confused with bimodality. Composers use mode mixing to freely borrow harmonies from a major key and its relative minor. This allows the composer to use more color and different types of progressions and chord relationships in the work, but the modes alternate rather than being present at the same time. This technique differs from bimodality in that the composer cannot use melodies or harmonies from both major and minor keys at the same time.