A cluster is a chord built by at least three adjacent tones of a scale. In this form they are similar to chords by seconds as the adjacent tones usually correspond to second intervals as clusters can be based on chromatic, diatonic or pentatonic scales. The main difference between these two chord formations is that a cluster is not inverted while a chord by seconds has that flexibility.
Normally you may hear clusters played harmonically but it may also be arpeggiated in a way that introduces the cluster by holding all the previous notes. However, it is advisable to do so in a harmonic cluster context so that when arpeggiated it doesn’t sound like a scale fragment:
Depending on the way that you choose to introduce the notes of a cluster, or the way certain parts of it are highlighted, it may sound more consonant or dissonant:
Clusters may also be built by using more than one chord at the time (see polychords), and arranged in a way that both chords are firstly introduced in their cluster or “un-clustered” form:
The same or different scales can be used to build a polycluster, meaning that more than one cluster is being used.
That said, it is wise to space them so that each cluster can be heard distinctly.
A polycluster may be arranged in seconds, thirds or fourths, and while the notes involved in the clusters themselves are not inverted, the cluster groups can be:
As previously stated, be wary of the registers in which the clusters are executed on the risk of sounding too weak (high register), or too muddy; although depending on the effect you are going for it might be fine. Just make sure it is a conscious decision.
Cluster harmony is generally not easy to handle regarding movement and harmonic functionality. Adding movement to cluster formations is usually achieved by working with the outer voices that are also used to contract or expand these formations.
Contracting chord formations using clusters with chord by fourths and thirds
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