Gunther Schuller, a composer, conductor, author and teacher who coined the term Third Stream to describe music that drew on the forms and resources of both classical and jazz, and who was its most important composer, died on Sunday in Boston. He was 89.

The cause was complications of leukemia, said his personal assistant, Jennique Horrigan.

Mr. Schuller, who won the Pulitzer Prize for his orchestral work “Of Reminiscences and Reflections” in 1994, was partial to the 12-tone methods of the Second Viennese School, but he was not inextricably bound to them. Always fascinated by jazz, he wrote arrangements as well as compositions for several jazz artists, most notably the Modern Jazz Quartet. Several of his scores — among them the Concertino (1958) for jazz quartet and orchestra, the “Seven Studies on Themes of Paul Klee” (1959) and an opera, “The Visitation” (1966) — used aspects of his Third Stream aesthetic, though usually with contemporary classical influences dominating.

Much of Mr. Schuller’s best music is scored for unusual instrumental combinations. In the Symphony for Brass and Percussion (1950), one of his most widely performed early works, he sent the strings and woodwinds to the sidelines. In “Spectra,” a study in orchestral color composed for the New York Philharmonic in 1960, he split the orchestra into seven distinct groups, deployed separately on the stage so that each could be heard independently or in combination with the others. He also composed “Five Pieces for Five Horns” (1952) and quartets for four double basses (1947) and four cellos (1958). His more than 20 concertos include showpieces for the double bass (1968), the contrabassoon (1978) and the alto saxophone (1983), as well as a Grand Concerto for Percussion and Keyboards (2005), for eight percussionists, a harpist and two keyboardists.

Some of his works were thorny and brash. But they could also be poetic and evocative. “Of Reminiscences and Reflections,” a rich, emotionally direct orchestral score, was composed as an elegy for Mr. Schuller’s wife, Marjorie, who died in November 1992. In his “Impromptus and Cadenzas” (1990), a chamber work, harmonic spikiness was offset by currents of lyricism and unpredictable shifts of mood and tone color.

As a composer, Mr. Schuller was self-taught. Although his career took him from the horn section of the Cincinnati Symphony and the pit of the Metropolitan Opera to a handful of influential positions — among them the presidency of the New England Conservatory and the artistic directorship of the Berkshire Music Center at Tanglewood — he once described himself as “a high school dropout without a single earned degree.”

That he made this comment in a speech before the American Society of University Composers, in March 1980, was typical of Mr. Schuller. In addition to being fiercely proud of his self-taught status, he had an iconoclastic streak, and had a busy sideline delivering jeremiads in which he railed against either his listeners’ approach to music making or the musical world in general.

He told the university composers, for example, that it was time to abandon intellectual complexity for its own sake, and to write music that audiences could embrace — this despite his own devotion to the 12-tone method, which many listeners regarded as the root of the audience’s estrangement.

Only a few months earlier, in June 1979, Mr. Schuller had caused a stir by greeting the students who had come to Tanglewood to study at the Berkshire Music Center with an address in which he excoriated orchestras, orchestral musicians, conductors and unions for creating a situation in which, as he put it, “joy has gone out of the faces of many of our musicians,” replaced by “apathy, cynicism, hatred of new music” and other ills. Some of his arguments found their way into a compilation of his essays, “Musings: The Musical Worlds of Gunther Schuller” (1986), and his 1997 book, “The Compleat Conductor.”

But if Mr. Schuller learned composition on his own, he approached it with a solid grounding in musical basics. His paternal grandfather had been a conductor and teacher in Germany, and his father, Arthur Schuller, had played the violin in Germany under Wilhelm Furtwängler. Arthur Schuller joined the New York Philharmonic as a violinist and violist in 1923 and remained with the orchestra until 1965, and he encouraged his son to take up the flute and the French horn on the grounds that woodwind and brass players were in shorter supply than string players.

“I was fortunate to have been born into a musical home,” Mr. Schuller told The New York Times in 1977. “My father played with the New York Philharmonic for 42 years, and he had a lot of scores. When I was 11 or 12, I began buying my own scores, and at 13 I became a rabid record collector. Then, of course, there was playing. All of those things were my teachers, and they all complemented each other.”

Gunther Alexander Schuller was born in Queens on Nov. 22, 1925, to Arthur Schuller and the former Elsie Bernartz. After attending a private school in Gebesee, Germany, from 1932 to 1936, he returned to New York and enrolled at the St. Thomas Church Choir School, where he studied music with T. Tertius Noble and sang as a boy soprano. He also began to study the flute and the French horn, and was engaged by the Philharmonic as a substitute hornist when he was 15. He attended Jamaica High School in Queens; during his high school years, he also studied music theory and counterpoint at the Manhattan School of Music.

In 1943, Mr. Schuller dropped his studies to take his first professional job, touring as a French hornist with the American Ballet Theater. That same year he became the principal hornist of the Cincinnati Symphony Orchestra, a position he held until 1945, when he moved back to New York and became the principal hornist at the Metropolitan Opera. He was already composing as well, and before he left Cincinnati he was the soloist in the premiere of his own First Horn Concerto (1945).

It was also in Cincinnati that Mr. Schuller became interested in jazz, primarily through the music of Duke Ellington, which he transcribed from recordings and arranged for the Cincinnati Pops. As a player he began living a double life in New York, performing at the Metropolitan Opera and in chamber music concerts, and in ensembles led by, among others, Miles Davis.

He also began to temper his concert music with jazz elements, and he wrote a series of works to perform with the jazz pianist John Lewis, with both the Modern Jazz Quartet and a larger ensemble, the Modern Jazz Society. Typically, in these collaborations, Lewis would lead a jazz ensemble augmented by strings or woodwinds, which Mr. Schuller conducted.

In 1957, Mr. Schuller began describing these classical-jazz hybrids as Third Stream music. An important early showcase for the concept was a concert in May 1960 at the Circle in the Square theater, in which the Contemporary String Quartet and a starry cast of jazz musicians — among them the pianist Bill Evans and his trio, the guitarist Barry Galbraith, the multi-reed player Eric Dolphy and the alto saxophonist Ornette Coleman (who died on Thursday) — played a sampling of Mr. Schuller’s Third Stream works. He continued to champion the notion of Third Stream music throughout his career, sometimes expanding its definition.

“The Third Stream movement,” he once said, “inspires composers, improvisers and players to work together toward the goal of a marriage of musics, whether ethnic or otherwise, that have been kept apart by the tastemakers — fusing them in a profound way. And I think it’s appropriate that this has happened in this country, because America is the original cultural melting pot.”

For about 15 years, Mr. Schuller balanced his performing and composing careers by composing all night after playing opera performances. But by 1959 his schedule had become too arduous, and he decided to give up performing to devote himself more fully to composition.

The vacuum created by giving up his playing job was quickly filled with other noncompositional activities. In 1962 he published his first book, “Horn Technique,” which quickly became a standard reference work and was revised in 1992. In 1963 he began directing “20th Century Innovations,” a new-music series that ran for several seasons at Carnegie (now Weill) Recital Hall. That summer he was appointed acting head of the composition faculty at Tanglewood, and he took over the department fully in 1965. He soon became a powerful force at Tanglewood, directing the Berkshire Music Center from 1970 to 1984.

After he resigned from Tanglewood, he started a summer festival in Sandpoint, Idaho. He was also the music director of the Spokane Symphony for the 1984-85 season, and he maintained relationships with several other ensembles, including the Pro Arte Chamber Orchestra in Boston, of which he was principal guest conductor, and the Smithsonian Jazz Masterworks Orchestra in Washington, of which he was co-director with David Baker.

Mr. Schuller’s teaching career began in 1950, when he joined the faculty of the Manhattan School of Music. He taught composition at Yale from 1964 to 1967, when he was appointed president of the New England Conservatory. During his decade in that position, he introduced jazz and Third Stream music as focuses of conservatory training.

His own research into jazz proved fruitful as well. His “Early Jazz: Its Roots and Musical Development” (1968) and “The Swing Era: The Development of Jazz, 1930-1945” (1989) are highly regarded histories, and his recording of Scott Joplin’s “Red Back Book,” with the New England Conservatory Ragtime Ensemble, won a Grammy in 1974 and helped start the ragtime revival of the mid-1970s.

In 1975 Mr. Schuller established his own publishing companies, Gun-Mar Music and Margun Music, and in 1981 he started a record label, GM Recordings. With these companies, he produced printed editions of everything from early music to jazz transcriptions and contemporary works, as well as a large catalog of recordings by classical and jazz players, among them the Kronos Quartet, the pianists Russell Sherman, Frederick Moyer and Ran Blake, the saxophonist Joe Lovano and the guitarist Jim Hall.

Mr. Schuller, who lived in Newton Centre, Mass., is survived by his sons, Edwin and George, both professional musicians. In addition to the Pulitzer Prize, Mr. Schuller was awarded a MacArthur Foundation grant in 1991; the William Schuman Award, from Columbia University, in 1989; a Jazz Masters Fellowship (for advocacy) from the National Endowment for the Arts in 2008; and a lifetime achievement medal from the MacDowell Colony this year. “As a composer and teacher,” the composer Augusta Read Thomas, the chairwoman of the selection committee for the MacDowell award, said at the time, “he has inspired generations of students, setting an example of discovery and experimentation.”

In 2011 he published an autobiography, “Gunther Schuller: A Life in Pursuit of Music and Beauty.” That same year, he was the subject of a tribute concert at Weill Recital Hall, featuring two works by Mr. Schuller and two by the young composer Mohammed Fairouz.

In a laudatory review of that concert for The Times, Zachary Woolfe wrote of Mr. Schuller, “He has, as Mr. Fairouz said in an onstage discussion, big ears.”

Originally published at www.nytimes.com