George Russell, a jazz composer, educator and musician whose theories led the way to radical changes in jazz in the 1950s and ’60s, died on Monday in the Jamaica Plain neighborhood of Boston. He was 86 and lived in Boston.

The cause was complications of Alzheimer’s disease, said his wife, Alice.

Though he largely operated behind the scenes and was never well known to the general public, Mr. Russell was a major figure in one of the most important developments in post-World War II jazz: the emergence of modal jazz, the first major harmonic change in the music after bebop.

Bebop, the modern style pioneered by Charlie Parker, Dizzy Gillespie and others, had introduced a new level of harmonic sophistication, based on rapidly moving cycles of dense and sometimes dissonant chords. Modal jazz, as popularized by Miles Davis and John Coltrane, sought to give musicians more freedom and to simplify the harmonic playing field by, in essence, replacing chords with scales as the primary basis for improvisation.

Mr. Russell explained the concept in great detail in a book that came to be considered the bible of modal jazz, “The Lydian Chromatic Concept of Tonal Organization for Improvisation.” Conceived during bouts with tuberculosis in the 1940s, the book was originally self-published in 1953 and published as a book six years later. A final revised edition was published as Volume 1 in 2001; Mr. Russell had been working on a second volume.