Scale Theory: Bimodality
Bimodality is the practice of simultaneously using two distinct pitch collections. This makes the key or tonal center more ambiguous, and therefore, ostensibly more interesting for listeners. Bimodality can create unique harmonization and push the tonal boundaries of a musical composition. It’s generally considered a contemporary method, but is also found in classical music, and is closely related to the concept of polytonality (the use of two or more keys at the same time).
Bimodality is a more general approach than bitonality (the use of just two keys simultaneously) since the “scales” involved are not necessarily traditional scales. For instance, if diatonic collections are used, their pitch centers do not have to be traditional major and minor scales.
Bimodality is not a simple change of key because it requires the two pitch collections to be played concurrently, rather than switching from one key to another. An example of bimodality is when two musicians play two different modes, such as the Lydian and the Dorian. If one person plays four measures in the Lydian while another person plays the Dorian for the same four measures, that is bimodality.
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