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. Composers such as Béla Bartók and, later, Lou Harrison and Karlheinz Stockhausen became proponents of the tone cluster, which feature in the work of many 21st-century classical composers. Tone clusters play a significant role, as well, in the work of free jazz musicians such as Cecil Taylor and Matthew Shipp.
In most Western music, tone clusters tend to be heard as dissonant. Clusters may be performed with almost any individual instrument on which three or more notes can be played simultaneously, as well as by most groups of instruments or voices. Keyboard instruments are particularly suited to the performance of tone clusters because it is relatively easy to play multiple notes in unison on them.
Music theory and classification 
Prototypical tone clusters are chords of three or more adjacent notes on a chromatic scale, that is, three or more adjacent pitches each separated by only a semitone. Three-note stacks based on diatonic and pentatonic scales are also, strictly speaking, tone clusters. However, these stacks involve intervals between notes greater than the half-tone gaps of the chromatic kind. This can readily be seen on a keyboard, where the pitch of each key is separated from the next by one semitone (visualizing the black keys as extending to the edge of the keyboard): Diatonic scales—conventionally played on the white keys—contain only two semitone intervals; the rest are full tones. In Western musical traditions, pentatonic scales—conventionally played on the black keys—are built entirely from intervals larger than a semitone. Commentators thus tend to identify diatonic and pentatonic stacks as "tone clusters" only when they consist of four or more successive notes in the scale. In standard Western classical music practice, all tone clusters are classifiable as secundal chords—that is, they are constructed from minor seconds (intervals of one semitone), major seconds (intervals of two semitones), or, in the case of certain pentatonic clusters, augmented seconds (intervals of three semitones). Stacks of adjacent microtonal pitches also constitute tone clusters.
In tone clusters, the notes are sounded fully and in unison, distinguishing them from ornamented figures involving acciaccaturas and the like. Their effect also tends to be different: where ornamentation is used to draw attention to the harmony or the relationship between harmony and melody, tone clusters are for the most part employed as independent sounds. While, by definition, the notes that form a cluster must sound at the same time, there is no requirement that they must all begin sounding at the same moment. For example, in R. Murray Schafer's choral Epitaph for Moonlight (1968), a tone cluster is constructed by dividing each choir section (soprano/alto/tenor/bass) into four parts. Each of the sixteen parts enters separately, humming a note one semitone lower than the note hummed by the previous part, until all sixteen are contributing to the cluster.
Tone clusters have generally been thought of as dissonant musical textures, and even defined as such. As noted by Alan Belkin, however, instrumental timbre can have a significant impact on their effect: "Clusters are quite aggressive on the organ, but soften enormously when played by strings (possibly because slight, continuous fluctuations of pitch in the latter provide some inner mobility)." In his first published work on the topic, Henry Cowell observed that a tone cluster is "more pleasing" and "acceptable to the ear if its outer limits form a consonant interval." Tone clusters also lend themselves to use in a percussive manner. Historically, they were sometimes discussed with a hint of disdain. One 1969 textbook defines the tone cluster as "an extra-harmonic clump of notes."
Notation and execution 
In his 1917 piece The Tides of Manaunaun, Cowell introduced a new notation for tone clusters on the piano and other keyboard instruments. In this notation, only the top and bottom notes of a cluster, connected by a single line or a pair of lines, are represented. This developed into the solid-bar style seen in the image on the right. Here, the first chord—stretching two octaves from D2 to D4—is a diatonic (so-called white-note) cluster, indicated by the natural sign below the staff. The second is a pentatonic (so-called black-note) cluster, indicated by the flat sign; a sharp sign would be required if the notes showing the limit of the cluster were spelled as sharps. A chromatic cluster—black and white keys together—is shown in this method by a solid bar with no sign at all. In scoring the large, dense clusters of the solo organ work Volumina in the early 1960s, György Ligeti, using graphical notation, blocked in whole sections of the keyboard.
The performance of keyboard tone clusters is widely considered an "extended technique"—large clusters require unusual playing methods often involving the fist, the flat of the hand, or the forearm. Thelonious Monk and Karlheinz Stockhausen each performed clusters with their elbows; Stockhausen developed a method for playing cluster glissandi with special gloves. Don Pullen would play moving clusters by rolling the backs of his hands over the keyboard. Boards of various dimension are sometimes employed, as in the Concord Sonata (ca. 1904–19) of Charles Ives; they can be weighted down to execute clusters of long duration. Several of Lou Harrison's scores call for the use of an "octave bar", crafted to facilitate high-speed keyboard cluster performance. Designed by Harrison with his partner William Colvig, the octave bar is
a flat wooden device approximately two inches high with a grip on top and sponge rubber on the bottom, with which the player strikes the keys. Its length spans an octave on a grand piano. The sponge rubber bottom is sculpted so that its ends are slightly lower than its center, making the outer tones of the octave sound with greater force than the intermediary pitches. The pianist can thus rush headlong through fearfully rapid passages, precisely spanning an octave at each blow.
Use in Western music 
Before the 1900s 
The earliest example of tone clusters in a Western music composition thus far identified is in the Allegro movement of Heinrich Biber's Battalia à 10 (1673) for string ensemble, which calls for several diatonic clusters. An orchestral diatonic cluster occurs also in the representation of chaos in the opening of Jean-Féry Rebel's 1737–38 ballet Les Elémens. From the next century-and-a-half, only a few more examples have been identified, none calling for more than a fleeting instance of the form. These are mostly in French programmatic compositions for the harpsichord or piano that represent cannon fire with clusters, for example in works by François Dandrieu (Les Caractères de la guerre, 1724), Michel Corrette (La Victoire d'un combat naval, remportée par une frégate contre plusieurs corsaires réunis, 1780), Claude-Bénigne Balbastre (March des Marseillois, 1793), Pierre Antoine César (La Battaille de Gemmap, ou la prise de Mons, ca. 1794), Bernard Viguerie (La Bataille de Maringo, pièce militaire et hitorique, for piano trio, 1800), and Jacques-Marie Beauvarlet-Charpentier (Battaille d'Austerlitz, 1805).
The next known composition after Charpentier's to feature several clusters—though not, in this case, specifically notated—is the solo piano piece Battle of Manassas, written in 1861 by "Blind Tom" Bethune and published in 1866. The score instructs the pianist to represent cannon fire at various points by striking "with the flat of the hand, as many notes as possible, and with as much force as possible, at the bass of the piano." In 1887, Giuseppe Verdi became the first notable composer in the Western tradition to write an unmistakable chromatic cluster: the storm music with which Otello opens includes an organ cluster (C, C♯, D) that also has the longest notated duration of any scored musical texture known. Still, it was not before the second decade of the twentieth century that tone clusters assumed a recognized place in Western classical music practice.
In classical music of the early 1900s 
"Around 1910," Harold C. Schoenberg writes, "Percy Grainger was causing a stir by the near–tone clusters in such works as his Gumsuckers March." In 1911, what appears to be the first published classical composition to thoroughly integrate true tone clusters was issued: Tintamarre (The Clangor of Bells), by Canadian composer J. Humfrey Anger (1862–1913).
Within a few years, the radical composer-pianist Leo Ornstein became one of the most famous figures in classical music on both sides of the Atlantic for his performances of cutting-edge work. In 1914, Ornstein debuted several of his own solo piano compositions: Wild Men's Dance (aka Danse Sauvage; ca. 1913–14), Impressions of the Thames (ca. 1913–14), and Impressions of Notre Dame (ca. 1913–14) were the first works to explore the tone cluster in depth ever heard by a substantial audience. Wild Men's Dance, in particular, was constructed almost entirely out of clusters ( listen (help·info)). In 1918, critic Charles L. Buchanan described Ornstein's innovation: "[He] gives us masses of shrill, hard dissonances, chords consisting of anywhere from eight to a dozen notes made up of half tones heaped one upon another."
Clusters were also beginning to appear in more pieces by European composers. Isaac Albéniz's use of them in Iberia (1905–8) may have influenced Gabriel Fauré's subsequent piano writing. Joseph Horowitz has suggested that the "dissonant star clusters" in its third and fourth books were particularly compelling to Olivier Messiaen, who called Iberia "the wonder of the piano." The Thomas de Hartmann score for Wassily Kandinsky's stage show The Yellow Sound (1909) employs a chromatic cluster at two climactic points. Alban Berg's Four Pieces for clarinet and piano (1913) calls for clusters along with other avant-garde keyboard techniques. Claude Debussy's 1915 arrangement for solo piano of his Six Epigraphes Antiques (1914), originally a set of piano duets, includes tone clusters in the fifth piece, Pour l'Egyptienne. Richard Strauss's An Alpine Symphony (1915) "starts and ends with the setting sun—a B flat minor chord cluster slowly built down."
Though much of his work was made public only years later, Charles Ives had been exploring the possibilities of the tone cluster—which he referred to as the "group chord"—for some time. In 1906–7, Ives composed his first mature piece to extensively feature tone clusters, Scherzo: Over the Pavements. Orchestrated for a nine-piece ensemble, it includes both black- and white-note clusters for the piano. Revised in 1913, it would not be recorded and published until the 1950s and would have to wait until 1963 to receive its first public performance. During the same period that Ornstein was introducing tone clusters to the concert stage, Ives was developing a piece with what would become the most famous set of clusters: in the second movement, "Hawthorne," of the Concord Sonata (ca. 1904–15, publ. 1920, prem. 1928, rev. 1947), mammoth piano chords require a wooden bar almost fifteen inches long to play. The gentle clusters produced by the felt- or flannel-covered bar represent the sound of far-off church bells ( listen (help·info)). Later in the movement, there are a series of five-note diatonic clusters for the right hand. In his notes to the score, Ives indicates that "these group-chords...may, if the player feels like it, be hit with the clenched fist." Between 1911 and 1913, Ives also wrote ensemble pieces with tone clusters such as his Second String Quartet and the orchestral Decoration Day and Fourth of July, though none of these would be publicly performed before the 1930s.
In the work of Henry Cowell 
In June 1913, a sixteen-year-old Californian with no formal musical training wrote a solo piano piece, Adventures in Harmony, employing "primitive tone clusters." Henry Cowell would soon emerge as the seminal figure in promoting the cluster harmonic technique. Ornstein abandoned the concert stage in the early 1920s and, anyway, clusters had served him as practical harmonic devices, not as part of a larger theoretical mission. In the case of Ives, clusters comprised a relatively small part of his compositional output, much of which went unheard for years. For the intellectually ambitious Cowell—who heard Ornstein perform in New York in 1916—clusters were crucial to the future of music. He set out to explore their "overall, cumulative, and often programmatic effects."
Dynamic Motion (1916) for solo piano, written when Cowell was nineteen, has been described as "probably the first piece anywhere using secundal chords independently for musical extension and variation." Though that is not quite accurate, it does appear to be the first piece to employ chromatic clusters in such a manner. A solo piano piece Cowell wrote the following year, The Tides of Manaunaun (1917), would prove to be his most popular work and the composition most responsible for establishing the tone cluster as a significant element in Western classical music ( listen (help·info)). (Cowell's early piano works are often erroneously dated; in the two cases above, as 1914 and 1912, respectively.) Assumed by some to involve an essentially random—or, more kindly, aleatoric—pianistic approach, Cowell would explain that precision is required in the writing and performance of tone clusters no less than with any other musical feature:
Tone clusters...on the piano [are] whole scales of tones used as chords, or at least three contiguous tones along a scale being used as a chord. And, at times, if these chords exceed the number of tones that you have fingers on your hand, it may be necessary to play these either with the flat of the hand or sometimes with the full forearm. This is not done from the standpoint of trying to devise a new piano technique, although it actually amounts to that, but rather because this is the only practicable method of playing such large chords. It should be obvious that these chords are exact and that one practices diligently in order to play them with the desired tone quality and to have them absolutely precise in nature.
Historian and critic Kyle Gann describes the broad range of ways in which Cowell constructed (and thus performed) his clusters and used them as musical textures, "sometimes with a top note brought out melodically, sometimes accompanying a left-hand melody in parallel."
Beginning in 1921, with an article serialized in The Freeman, an Irish cultural journal, Cowell popularized the term tone cluster. While he did not coin the phrase, as is often claimed, he appears to have been the first to use it with its current meaning. During the 1920s and 1930s, Cowell toured widely through North America and Europe, playing his own experimental works, many built around tone clusters. In addition to The Tides of Manaunaun, Dynamic Motion, and its five "encores"—What's This (1917), Amiable Conversation (1917), Advertisement (1917), Antinomy (1917, rev. 1959; frequently misspelled "Antimony"), and Time Table (1917)—these include The Voice of Lir (1920), Exultation (1921), The Harp of Life (1924), Snows of Fujiyama (1924), Lilt of the Reel (1930), and Deep Color (1938). Tiger (1930) has a chord of 53 notes, probably the largest ever written for a single instrument until 1969. Along with Ives, Cowell wrote some of the first large-ensemble pieces to make extensive use of clusters. The Birth of Motion (ca. 1920), his earliest such effort, combines orchestral clusters with glissando. "Tone Cluster," the second movement of Cowell's Concerto for Piano and Orchestra (1928, prem. 1978), employs a wide variety of clusters for the piano and each instrumental group ( listen (help·info)). From a quarter-century later, his Symphony No. 11 (1953) features a sliding chromatic cluster played by muted violins.
In his theoretical work New Musical Resources (1930), a major influence on the classical avant-garde for many decades, Cowell argued that clusters should not be employed simply for color:
In harmony it is often better for the sake of consistency to maintain a whole succession of clusters, once they are begun; since one alone, or even two, may be heard as a mere effect, rather than as an independent and significant procedure, carried with musical logic to its inevitable conclusion.
In later classical music 
In 1922, composer Dane Rudhyar, a friend of Cowell's, declared approvingly that the development of the tone cluster "imperilled [the] existence" of "the musical unit, the note." While that threat was not to be realized, clusters began to appear in the works of a growing number of composers. Already, Aaron Copland had written his Three Moods (aka Trois Esquisses; 1920–21) for piano—its name an apparent homage to a piece of Leo Ornstein's—which includes a triple-forte cluster. The most renowned composer to be directly inspired by Cowell's demonstrations of his tone cluster pieces was Béla Bartók, who requested Cowell's permission to employ the method. Bartók's First Piano Concerto, Piano Sonata, and Out of Doors (all 1926), his first significant works after three years in which he produced little, extensively feature tone clusters.
In the 1930s, Cowell's student Lou Harrison utilized keyboard clusters in several works such as his Prelude for Grandpiano (1937). At least as far back as 1942, John Cage, who also studied under Cowell, began writing piano pieces with cluster chords; In the Name of the Holocaust, from December of that year, includes chromatic, diatonic, and pentatonic clusters. Olivier Messiaen's Vingt regards sur l'enfant Jésus (1944), often described as the most important solo piano piece of the first half of the twentieth century, employs clusters throughout. They would feature in numerous subsequent piano works, by a range of composers. Karlheinz Stockhausen's Klavierstück X (1961) makes bold, rhetorical use of chromatic clusters, scaled in seven degrees of width, from three to thirty-six semitones, as well as ascending and descending cluster arpeggios and cluster glissandi. Written two decades later, his Klavierstück XIII employs many of the same techniques, along with clusters that call for the pianist to sit down on the keyboard. George Crumb's Apparitions, Elegiac Songs, and Vocalises for Soprano and Amplified Piano (1979), a setting of verse by Walt Whitman, is filled with clusters, including an enormous one that introduces three of its sections. The piano part of the second movement of Joseph Schwantner's song cycle Magabunda (1983) has perhaps the single largest chord ever written for an individual instrument: all 88 notes on the keyboard.
While tone clusters are conventionally associated with the piano, and the solo piano repertoire in particular, they have also assumed important roles in compositions for chamber groups and larger ensembles. Robert Reigle identifies Croatian composer Josip Slavenski's organ-and-violin Sonata Religiosa (1925), with its sustained chromatic clusters, as "a missing link between Ives and [György] Ligeti." Bartók employs both diatonic and chromatic clusters in his Fourth String Quartet (1928). The sound mass technique in such works as Ruth Crawford Seeger's String Quartet (1931) and Iannis Xenakis's Metastaseis (1955) is an elaboration of the tone cluster. In one of the most famous pieces associated with the sound mass aesthetic, Krzysztof Penderecki's Threnody to the Victims of Hiroshima (1959), for fifty-two string instruments, the quarter-tone clusters "see[m] to have abstracted and intensified the features that define shrieks of terror and keening cries of sorrow." Clusters appear in two sections of the electronic music of Stockhausen's Kontakte (1958–60)—first as "hammering points...very difficult to synthesize", according to Robin Maconie, then as glissandi. In 1961, Ligeti wrote perhaps the largest cluster chord ever—in the orchestral Atmosphères, every note in the chromatic scale over a range of five octaves is played at once (quietly). Ligeti's organ works make extensive use of clusters. Volumina (1961–62), graphically notated, consists of static and mobile cluster masses, and calls on many advanced cluster-playing techniques.
The eighth movement of Messiaen's oratorio La Transfiguration de Notre Seigneur Jésus-Christ (1965–69) features "a shimmering halo of tone-cluster glissandi" in the strings, evoking the "bright cloud" to which the narrative refers ( listen (help·info)). Orchestral clusters are employed throughout Stockhausen's Fresco (1969) and Trans (1971). In Morton Feldman's Rothko Chapel (1971), "Wordless vocal tone clusters seep out through the skeletal arrangements of viola, celeste, and percussion." Aldo Clementi's chamber ensemble piece Ceremonial (1973) evokes both Verdi and Ives, combining the original extended-duration and mass cluster concepts: a weighted wooden board placed on an electric harmonium maintains a tone cluster throughout the work. Judith Bingham's Prague (1995) gives a brass band the opportunity to create tone clusters. Keyboard clusters are set against orchestral forces in piano concertos such as Einojuhani Rautavaara's first (1969) and Esa-Pekka Salonen's (2007), the latter suggestive of Messiaen. The choral compositions of Eric Whitacre often employ clusters.
Three composers who made frequent use of tone clusters for a wide variety of ensembles are Giacinto Scelsi, Alfred Schnittke—both of whom often worked with them in microtonal contexts—and Lou Harrison. Scelsi employed them for much of his career, including in his last large-scale work, Pfhat (1974), which premiered in 1986. They are found in works of Schnittke's ranging from the Quintet for Piano and Strings (1972–76), where "microtonal strings fin[d] tone clusters between the cracks of the piano keys," to the choral Psalms of Repentance (1988). Harrison's many pieces featuring clusters include Pacifika Rondo (1963), Concerto for Organ with Percussion (1973), Piano Concerto (1983–85), Three Songs for male chorus (1985), Grand Duo (1988), and Rhymes with Silver (1996).
In jazz 
Tone clusters have been employed by jazz artists in a variety of styles, since the very beginning of the form. Around the turn of the twentieth century, Storyville pianist Jelly Roll Morton began performing a ragtime adaptation of a French quadrille, introducing large chromatic tone clusters played by his left forearm. The growling effect led Morton to dub the piece his "Tiger Rag" ( listen (help·info)). In 1909, Scott Joplin's deliberately experimental "Wall Street Rag" included a section prominently featuring notated tone clusters—apparently the first published work in the history of Western music with a cluster sequence. The fourth of Artie Matthews's Pastime Rags (1913–20) features dissonant right-hand clusters. Thelonious Monk, in pieces such as "Introspection" (1946) and "Off Minor" (1947), uses clusters as dramatic figures within the central improvisation and to accent the tension at its conclusion. They are heard on Art Tatum's "Mr. Freddy Blues" (1950), undergirding the cross-rhythms. By 1953, Dave Brubeck was employing piano tone clusters and dissonance in a manner anticipating the style free jazz pioneer Cecil Taylor would soon develop. The approach of hard bop pianist Horace Silver is an even clearer antecedent to Taylor's use of clusters. During the same era, clusters appear as punctuation marks in the lead lines of Herbie Nichols. In "The Gig" (1955), described by Francis Davis as Nichols's masterpiece, "clashing notes and tone clusters depic[t] a pickup band at odds with itself about what to play." Recorded examples of Duke Ellington's piano cluster work include "Summertime" (1961) and ...And His Mother Called Him Bill (1967).
In jazz, as in classical music, tone clusters have not been restricted to the keyboard. In the 1930s, the Jimmie Lunceford Orchestra's "Stratosphere" included ensemble clusters among an array of progressive elements. The Stan Kenton Orchestra's April 1947 recording of "If I Could Be With You One Hour Tonight," arranged by Pete Rugolo, features a dramatic four-note trombone cluster at the end of the second chorus. As described by critic Fred Kaplan, a 1950 performance by the Duke Ellington Orchestra features arrangements with the collective "blowing rich, dark, tone clusters that evoke Ravel." In the early 1960s, arrangements by Bob Brookmeyer and Gerry Mulligan for Mulligan's Concert Jazz Band employed tone clusters in a dense style bringing to mind both Ellington and Ravel. Eric Dolphy's bass clarinet solos would often feature "microtonal clusters summoned by frantic overblowing." Critic Robert Palmer called the "tart tone cluster" that "pierces a song's surfaces and penetrates to its heart" a specialty of guitarist Jim Hall's.
Clusters are especially prevalent in the realm of free jazz. Cecil Taylor has used them extensively as part of his improvisational method since the mid-1950s. Like much of his musical vocabulary, his clusters operate "on a continuum somewhere between melody and percussion." One of Taylor's primary purposes in adopting clusters was to avoid the dominance of any specific pitch. Leading free jazz composer, bandleader, and pianist Sun Ra often used them to rearrange the musical furniture, as described by scholar John F. Szwed:
When he sensed that [a] piece needed an introduction or an ending, a new direction or fresh material, he would call for a space chord, a collectively improvised tone cluster at high volume which "would suggest a new melody, maybe a rhythm." It was a pianistically conceived device which created another context for the music, a new mood, opening up fresh tonal areas.
As free jazz spread in the 1960s, so did the use of tone clusters. In comparison with what John Litweiler describes as Taylor's "endless forms and contrasts," the solos of Muhal Richard Abrams employ tone clusters in a similarly free, but more lyrical, flowing context. Guitarist Sonny Sharrock made them a central part of his improvisations; in Palmer's description, he executed "glass-shattering tone clusters that sounded like someone was ripping the pickups out of the guitar without having bothered to unplug it from its overdriven amplifier." Pianist Marilyn Crispell has been another major free jazz proponent of the tone cluster, frequently in collaboration with Anthony Braxton, who played with Abrams early in his career. Since the 1990s, Matthew Shipp has built on Taylor's innovations with the form. European free jazz pianists who have contributed to the development of the tone cluster palette include Gunter Hampel and Alexander von Schlippenbach.
Don Pullen, who bridged free and mainstream jazz, "had a technique of rolling his wrists as he improvised—the outside edges of his hands became scarred from it—to create moving tone clusters," writes critic Ben Ratliff. "Building up from arpeggios, he could create eddies of noise on the keyboard...like concise Cecil Taylor outbursts." In the description of Joachim Berendt, Pullen "uniquely melodized cluster playing and made it tonal. He phrases impulsively raw clusters with his right hand and yet embeds them in clear, harmonically functional tonal chords simultaneously played with the left hand." John Medeski employs tone clusters as keyboardist for Medeski, Martin, and Wood, which mixes free jazz elements into its soul jazz/jam band style.
In popular music 
Like jazz, rock and roll has made use of tone clusters since its birth, if characteristically in a less deliberate manner—most famously, Jerry Lee Lewis's live-performance piano technique of the 1950s, involving fists, feet, and derrière. Since the 1960s, much drone music, which crosses the lines between rock, electronic, and experimental music, has been based on tone clusters. On The Velvet Underground's "Sister Ray," recorded in September 1967, organist John Cale uses tone clusters within the context of a drone; the song is apparently the closest approximation on record of the band's early live sound. Around the same time, Doors keyboardist Ray Manzarek began introducing clusters into his solos during live performances of the band's hit "Light My Fire."
Kraftwerk's self-titled 1970 debut album employs organ clusters to add variety to its repeated tape sequences. In 1971, critic Ed Ward lauded the "tone-cluster vocal harmonies" created by Jefferson Airplane's three lead singers, Grace Slick, Marty Balin, and Paul Kantner. Tangerine Dream's 1972 double album Zeit is replete with clusters performed on synthesizer. In later rock practice, the D add9 chord characteristic of jangle pop involves a three-note set separated by major seconds (D, E, F♯), the sort of guitar cluster that may be characterized as a harp effect.
The sound of tone clusters played on the organ became a convention in radio drama for dreams. Clusters are often used in the scoring of horror and science-fiction films. For a 2004 production of the play Tone Clusters by Joyce Carol Oates, composer Jay Clarke—a member of the indie rock bands Dolorean and The Standard—employed clusters to "subtly build the tension", in contrast to what he perceived in the cluster pieces by Cowell and Ives suggested by Oates: “Some of it was like music to murder somebody to; it was like horror-movie music”.
Use in other music 
In traditional Japanese gagaku, the imperial court music, a tone cluster performed on shō (a type of mouth organ) is generally employed as a harmonic matrix. Yoritsune Matsudaira, active from the late 1920s to the early 2000s, merged gagaku's harmonies and tonalities with avant-garde Western techniques. Much of his work is built on the shō's ten traditional cluster formations. Lou Harrison's Pacifika Rondo, which mixes Eastern and Western instrumentation and styles, mirrors the gagaku approach—sustained organ clusters emulate the sound and function of the shō. Traditional Korean court and aristocratic music employs passages of simultaneous ornamentation on multiple instruments, creating dissonant clusters; this technique is reflected in the work of twentieth-century Korean German composer Isang Yun.
Several East Asian free reed instruments, including the shō, were modeled on the sheng, an ancient Chinese folk instrument later incorporated into more formal musical contexts. Wubaduhesheng, one of the traditional chord formations played on the sheng, involves a three-pitch cluster. Malayan folk musicians employ an indigenous mouth organ that, like the shō and sheng, produces tone clusters. The characteristic musical form played on the bin-baja, a strummed harp of central India's Pardhan people, has been described as a "rhythmic ostinato on a tone cluster."
- See Nicholls (1991), p. 155.
- Jones (2008), p. 91; Wilkins (2006), p. 145; Norman (2004), p. 47.
- Cope (2001), p. 6, fig. 1.17.
- Swift (1972), pp. 511–12.
- See, e.g., Seachrist (2003), p. 215, n. 15: "A 'tone cluster' is a dissonant group of tones lying close together...."
- Belkin, Alan (2003). "Harmony and Texture; Orchestration and Harmony/Timbre". Université de Montréal. Archived from the original on 2007-06-22. Retrieved 2007-08-18.
- Cowell (1921), pp. 112, 113.
- Ostransky (1969), p. 208.
- The score of The Tides of Manaunaun is reprinted in American Piano Classics: 39 Works by Gottschalk, Griffes, Gershwin, Copland, and Others, ed. Joseph Smith (Mineola, N.Y.: Courier Dover, 2001; ISBN 0-486-41377-2), pp. 43 et seq.
- Hicks (2002), p. 103.
- Griffiths (1995), p. 137.
- Tyranny, "Blue" Gene (2003-10-01). "88 Keys to Freedom: Segues Through the History of American Piano Music—The Keyboard Goes Bop! and the Melody Spins Off into Eternity (1939 to 1952)". NewMusicBox. American Music Center. Retrieved 2007-08-20. Cooke (1998), p. 205.
- Ratliff (2002), p. 205.
- Hinson and Roberts (2006), p. 624.
- Miller and Lieberman (2004), p. 135.
- "Earliest Usages: 1. Pitch" in Byrd, Donald (2010-05-12). "Extremes of Conventional Music Notation". Indiana University, School of Informatics (website material based in part on work supported by the National Science Foundation). Retrieved 2011-08-29.
- Henck (2004), pp. 52–54.
- Henck (2004), pp. 32–40.
- Quoted in Altman (2004), p. 47.
- Kimbell (1991), p. 606; "Earliest Usages: 1. Pitch" and "Duration and Rhythm: 2. Longest notated duration, including ties" in Extremes of Conventional Music Notation.
- Schoenberg (1987), p. 419.
- For a discussion of the piece, see Keillor, Elaine (2004-05-10). "Writing for a Market—Canadian Musical Composition Before the First World War". Library and Archives Canada/Bibliothèque et Archives Canada. Retrieved 2011-08-29. The score of Tintamarre and its publication record are also available online via Library and Archives Canada/Bibliothèque et Archives Canada. See also Keillor (2000) and "Anger, Humfrey". Encyclopedia of Music in Canada. The Canadian Encyclopedia. Retrieved 2007-08-20. The early performance history of Tintamarre has not been established.
- See Broyles (2004), p. 78, for premiere of these works. The piano music for Ornstein's Sonata for Violin and Piano, op. 31 (1915; not 1913 as is often erroneously given), also employs tone clusters, though not to the extent of Wild Men's Dance. Three Moods (ca. 1914) for solo piano has been said to contain clusters (Pollack , p. 44); perusal online of the published score, however, does not reveal any. Ornstein's solo piano piece Suicide in an Airplane (n.d.), which makes incontrovertible use of tone clusters in one extended passage, is often erroneously dated "1913" or "ca. 1913"; in fact, it is undated and there is no record of its existence before 1919 (Anderson ).
- Quoted in Chase (1992), p. 450.
- Nectoux (2004), p. 171.
- Horowitz (2010), p. 18.
- Finney (1967), p. 74.
- Pino (1998), p. 258.
- Hinson (1990), pp. 43–44.
- W.S.M. (1958), p. 63.
- Thomas B. Holmes notes that the song Majority (aka The Masses), written by Ives in 1888 at the age of fourteen, incorporates tone clusters in the piano accompaniment. He correctly describes this as "a rebellious act for a beginning composer." He errs in calling it "probably the first documented use of a tone cluster in a score" (Electronic and Experimental Music: Pioneers in Technology and Composition [New York and London: Routledge, 2002 (1985); ISBN 0-415-93643-8], p. 35). Swafford (1998) observes that Ives chose to begin his 114 Songs (publ. 1922) with the work (pp. 227, 271, 325). And he too miscredits Ives with the "invention of the tone cluster" (p. 231). On the other hand, he valuably points to Ives's awareness that "tone clusters...had been there since time immemorial when large groups sang. The mistakes were part of the music" (p. 98).
- Nicholls (1991), p. 57.
- Reed (2005), p. 59; Swafford (1998), p. 262. The "Hawthorne" movement was based on the unfinished Hawthorne Concerto of 1910, from which it was recomposed largely in 1911–12. See Ives (1947), p. iii.
- Shreffler (1991), p. 3; Hitchcock (2004), p. 2.
- Ives (1947), p. 73. Ives's orthography was not consistent. When the term "group chord" is introduced earlier in the notes, it appears without a hyphen.
- Swafford (1998), pp. 251, 252, 472, for descriptions; Sinclair (1999), passim, for proper dating of Scherzo: Over the Pavements, Concord Sonata, and other named pieces: Second String Quartet (1911–13, prem. 1946, publ. 1954); Decoration Day (ca. 1912–13, rev. ca. 1923–24, prem. 1931, publ. 1962); Fourth of July (ca. 1911–13, rev. ca. 1931, publ./prem. 1932).
- Nicholls (1991), p. 134.
- Broyles (2004), p. 342, n. 10.
- Bartok et al. (1963), p. 14 (unpaginated).
- Correct dating of Cowell's early works is per Hicks (2002), pp. 80, 85. Correct dating of Cowell's work in general is per the standard catalogue, Lichtenwanger (1986).
- Cowell (1993), 12:16–13:14.
- Gann, Kyle (2003-10-21). "Rosen's Sins of American Omission". ArtsJournal. Retrieved 2007-08-18.
- Hicks (2002), pp. 106–8.
- See Seachrist (2003), p. 215, n. 15, for an example of a claim that the "term was invented by Henry Cowell." Tone cluster had been used with a different meaning since at least 1910 by music theorist and educator Percy Goetschius: referring to an example of three-part counterpoint, "there is some good chord-form at almost every accent, some harmonic tone-cluster towards which the parts unanimously lead" (Exercises in Elementary Counterpoint, 5th ed. [New York: G. Schirmer], p. 111). See also his correspondence, "Schoenberg's 'Harmony,'" in The New Music Review and Church Music Review, vol. 14, no. 168 (November 1915), p. 404: "I have regretted that I did not, in revising my 'Material,' lay still greater stress upon the accidental tone-clusters such as you illustrate"; "in Ex. 318, No. 5, you will find the Mozart tone-cluster which you give in your Ex. 11."
- "Other: 1. Vertical extremes" in Extremes of Conventional Music Notation.
- Yunwha Rao (2004), p. 245.
- Zwenzner (2001), p. 13.
- Yunwha Rao (2004), p. 138.
- Quoted in Gann (1997), p. 174.
- Quoted in Hicks (2002), p. 108.
- Pollack (2000), p. 44.
- Stevens (1993), p. 67.
- Steinberg (2000), p. 37; Satola (2005), pp. 85–86; Lampert and Somfai (1984), p. 60.
- Miller and Lieberman (2004), pp. 10, 135.
- Salzman (1996), p. 3 (unpaginated).
- Meister (2006), p. 131–32.
- Harvey (1975), p. 43; Henck (1980), p. 17; Maconie (2005), p. 217.
- Rigoni (2001), p. 53.
- Kramer (2000), p. 137.
- "Other: 1. Vertical extremes" in Extremes of Conventional Music Notation.
- Reigle, Robert (April 2002). "Forgotten Gems". La Folia. Retrieved 2007-08-19.
- Trueman, Daniel (1999). "Three "Classical" Violins and a Fiddle" (PDF). Reinventing the Violin. Princeton University, Department of Music. Retrieved 2007-08-19. See also Robin Stowell, "Extending the Technical and Expressive Frontiers," in The Cambridge Companion to the String Quartet, ed. Stowell (Cambridge: Cambridge University Press, 2003; ISBN 0-521-80194-X), pp. 149–73; p. 162.
- Hogan (2003), p. 179.
- Maconie (2005), p. 217.
- Steinitz (2003), pp. 124–26; Herchenröder (2002), p. 303.
- Smither (2000), p. 674.
- Maconie (2005), p. 338.
- Swan, Glenn. "Morton Feldman: Rothko Chapel (1971) for Chorus, Viola and Percussion/Why Patterns? (1978) (review)". Allmusic. WindowsMedia. Archived from the original on September 30, 2007. Retrieved 2007-08-18.
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- Tommasini (2007).
- "Brigham Young Univ. Singers: Eric Whitacre: Complete A Cappella Works (review)". a-capella.com. 2002. Archived from the original on 2008-02-09. Retrieved 2008-02-25. Paulin, Scott (2006). "Whitacre: Cloudburst and Other Choral Works (review)". Barnes & Noble. Retrieved 2008-02-25.
- Halbreich (1988), pp. 9, 11 (unpaginated).
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- Miller and Lieberman (2004), pp. 10, 99, 135, 155.
- Lomax (2001), pp. 66–69; Spaeth (1948), p. 420.
- See Floyd (1995), p. 72; Berlin (1994), p. 187.
- Magee (1998), p. 402.
- Meadows (2003), ch. 10.
- Harrison (1997), p. 315.
- "Jazz Legends: Dave Brubeck". Jazz Improv. Archived from the original on April 18, 2007. Retrieved 2007-08-18.
- Hazell (1997); Litweiler (1990), p. 202. See also Watrous (1989).
- Litweiler (1990), p. 23.
- Davis (2004), p. 78.
- Blumenfeld, Larry/Szwed, John F. (ed. Gary Giddins) (June 15, 1999). "Rockin' in Rhythm". Village Voice. Retrieved 2009-06-03.
- Determeyer (2006), p. 78.
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- Weinstein (1993), p. 84.
- Palmer (1986).
- Litweiler (1990), p. 202. See also Anderson (2006), pp. 57–58.
- Pareles (1988).
- Anderson (2006), p. 111.
- Szwed (1998), p. 214.
- Litweiler (1990), p. 182.
- Palmer (1991).
- Enstice and Stockhouse (2004), p. 81.
- Weinstein (1996).
- Svirchev, Laurence (2006). ""If You Start from Point-Zero, You Have to Imagine Something": An Interview with Alexander von Schlippenbach". Jazz Journalists Association Library. Retrieved 2007-08-17.
- Berendt (1992), p. 287.
- Pareles (2000).
- Tyler (2008), p. 76; Morrison (1998), p. 95.
- Schwartz (1996), pp. 97, 94.
- Hicks (1999), p. 88.
- Bussy (2004), p. 31.
- Quoted in Brackett (2002), p. 217.
- Patterson (2001), p. 505.
- Rooksby (2003), pp 18–19, 96.
- For a discussion of the use of tone clusters in film scoring, see David Huckvale, "Twins of Evil: An Investigation into the Aesthetics of Film Music," Popular Music, vol. 9, no. 1 (January 1990), pp. 1–35. For descriptions of their role in three individual films, see Shuhei Hosokawa, "Atomic Overtones and Primitive Undertones: Akira Ifukube's Sound Design for Godzilla," in Off the Planet: Music, Sound and Science Fiction Cinema, ed. Philip Hayward (Eastleigh, UK: John Libbey Publ., 2004; ISBN 0-86196-644-9), pp. 42–60; n. 21, p. 60; for To the Devil a Daughter, David Huckvale, Hammer Film Scores and the Musical Avant-Garde (Jefferson, N.C.: McFarland, 2008; ISBN 978-0-7864-3456-5), pp. 179–81 ; and, for Close Encounters, Neil Lerner, "Nostalgia, Masculinist Discourse, and Authoritarianism in John Williams' Scores for Star Wars and Close Encounters of the Third Kind," in Off the Planet, pp. 96–107; 105–6.
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- Musical Courier 164 (1962), p. 12.
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