Thelonious Sphere Monk, 63, the enigmatic composer, pianist and innovator who was one of the seminal figures of modern jazz, died yesterday at Englewood Hospital in Englewood, N.J. He had been hospitalized since suffering a stroke on Feb. 5.

Although he had been in ill health for many years and had been inactive through most of the 1970s -- he last appeared in concert at the Newport Jazz Festival in 1975 -- Mr. Monk was looked upon as a founding father by all segments of the jazz community from bop to the current avant-garde. His influence can be heard particularly in the playing of Cecil Taylor and Randy Weston.

Inspiring such major jazz voices as Dizzy Gillespie, Kenny Clarke, Charlie Parker, Miles Davis and Bud Powell, Mr. Monk helped initiate the harmonic and rhythmic innovations that led to a new style of jazz known as bebop.

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He was a prolific composer who contributed such notable jazz standards as "'Round Midnight," "Blue Monk," "Ruby, My Dear," "Straight, No Chaser" and "Epistrophy." He also composed the score for the French film, "Les Liaisons Dangereuses."

During his career, the reclusive Mr. Monk fostered a reputation as a true eccentric. He tended to wear dark glasses and an astounding array of hats. He seemed to have little concern with the strictures of time, often showing up at club dates hours late. Sometimes he would sway, trance-like, at the corner of the bandstand, listening to his soloists struggle with his melodies.

In his old New York neighborhood, he was known to stand on a street corner for hours, thinking about music. His neighbors, knowing his proclivities, never disturbed him.

Mr. Monk was born in Rocky Mount, N.C., and grew up in New York City. Despite several years of private piano lessons, his angular and iconoclastic technique seemed home-made and unbounded by convention.

In his teens, he played organ in a local church and also spent two years touring the country backing up evangelist singers. His first significant jazz experience came in 1940 when he joined the house band at Minton's Playhouse, an important meeting ground for musicians and an incubator for the bop movement.

Technically, Mr. Monk played incorrectly, with his fingers held parallel to the keyboard. His use of unusual intervals, dissonance and rhythmic displacement, coupled with a reputation for sardonic eccentricity, kept him in comparative obscurity until the early 1950s and the flowering of the hard bop movement.

In 1951, Mr. Monk's career suffered a serious setback when he was charged with possession of marijuana and lost his cabaret card until 1958. Without the card, he was unable to perform in New York City. The pianist was barely recorded during the early 1950s until a new progressive company under the Riverside label purchased his contract from his previous record company for slightly over $100.

Although Mr. Monk was a member of the Lucky Millinder orchestra in 1942 (with Gillespie), most of his career was spent in solo performance or as leader of trios and small ensembles. The most notable were a 1957 quartet featuring John Coltrane and another with ex-Washingtonian Charlie Rouse.

In February 1959, however, Mr. Monk led a large orchestra in a highly acclaimed concert at Town Hall in New York. Other concerts featuring large ensembles playing Monk compositions filled Philharmonic Hall in 1963 and Carnegie Hall in 1964. In 1964, he was pictured on the cover of Time magazine.

In the late 1950s, Mr. Monk, who adopted his middle name to indicate he wasn't "square," recorded a series of extraordinary albums, including "Brilliant Corners," "Criss Cross," "Misterioso," "5 by 5" and "Thelonious In Action."

Mr. Monk's compositions, though built upon traditional 12-bar blues and 32-bar ballad forms, were stripped to essentials and so cunningly constructed that they presented an immense challenge to even the best soloists. John Coltrane once said that "when you learn one of Monk's pieces, you can't just learn the melody and chord symbols. You have to learn the inner voicings and rhythms exactly. Everything is so carefully related; his works are jazz compositions in the sense that relatively few jazz 'originals' are."

Leonard Feather, writing in the Encyclopedia of Jazz, said that Mr. Monk "extended his mastery of an individual technique to the point where his harmonic innovations, coupled with the stark, somber quality of his approach and the uniquely subtle use of dynamics, place him among the most important and influential figures" in modern jazz.

Mr. Monk, who had lived in Weehawken, N.J., in recent years, is survived by his wife, Nellie, a son, Thelonious Sphere Jr., and a daughter, Barbara, who uses the professional name Boo Boo. Both children perform with a jazz-rock group that calls itself T.S. Monk.