Lou Harrison in the context of "Harry Partch"

A tone cluster is a musical chord comprising at least three adjacent tones in a scale. Prototypical tone clusters are based on the chromatic scale and are separated by semitones. For instance, three adjacent piano keys (such as C, C♯, and D) struck simultaneously produce a tone cluster. Variants of the tone cluster include chords comprising adjacent tones separated diatonically, pentatonically, or microtonally. On the piano, such clusters often involve the simultaneous striking of neighboring white or black keys.

The early years of the twentieth century saw tone clusters elevated to central roles in pioneering works by ragtime artists Jelly Roll Morton and Scott Joplin. In the 1910s, two classical avant-gardists, composer-pianists Leo Ornstein and Henry Cowell, were recognized as making the first extensive explorations of the tone cluster. During the same period, Charles Ives employed them in several compositions that were not publicly performed until the late 1920s or 1930s, as did Béla Bartók in the latter decade. Since the mid-20th century, they have prominently featured in the work of composers such as Lou Harrison, Giacinto Scelsi, Alfred Schnittke and Karlheinz Stockhausen, and later Eric Whitacre. Tone clusters also play a significant role in the work of free jazz musicians such as Cecil Taylor, Matthew Shipp, and Kevin Kastning.

Charles Edward Ives (/aɪvz/; October 20, 1874 – May 19, 1954) was an American modernist composer, actuary and businessman. Ives was among the earliest renowned American composers to achieve recognition on a global scale. His music was largely ignored during his early career, and many of his works went unperformed for many years. Later in life, the quality of his music was publicly recognized through the efforts of contemporaries like Henry Cowell and Lou Harrison, and he came to be regarded as an "American original". He was also among the first composers to engage in a systematic program of experimental music, with musical techniques including polytonality, polyrhythm, tone clusters, aleatory elements, and quarter tones. His experimentation foreshadowed many musical innovations that were later more widely adopted during the 20th century. Hence, he is often regarded as the leading American composer of art music of the 20th century.

Sources of Ives's tonal imagery included hymn tunes and traditional songs; he also incorporated melodies of the town band at holiday parade, the fiddlers at Saturday night dances, patriotic songs, sentimental parlor ballads, and the melodies of Stephen Foster.

Henry Dixon Cowell (/ˈkaʊəl/; March 11, 1897 – December 10, 1965) was an American composer, writer, pianist, publisher, teacher and the husband of Sidney Robertson Cowell. Earning a reputation as an extremely controversial performer and eccentric composer, Cowell became a leading figure of American avant-garde music for the first half of the 20th century — his writings and music serving as a great influence to similar artists at the time, including Lou Harrison, George Antheil, and John Cage, among others. He is considered one of America's most important and influential composers.

Cowell was mostly self-taught and developed a unique musical language, often blending folk melodies, dissonant counterpoint, unconventional orchestration, and themes of Irish paganism. He was an early proponent and innovator of many modernist compositional techniques and sensibilities, many for the piano, including the string piano, prepared piano, tone clusters, and graphic notation. The Tides of Manaunaun, originally a theatrical prelude, is the best-known and most widely-performed of Cowell's tone cluster pieces for piano.