Tone cluster

A group of several adjacent notes played simultaneously. Usually cluster chords are based on the chromatic scale but the notes can be also separated with whole tones or microintervals. Early examples of clusters can be found in works of Charles Ives, Henry Cowell, and Bela Bartók. Orchestral clusters were used by e.g. Iannis Xenakis, Krzysztof Penderecki, Karlheinz Stockhausen, and György Ligeti.