Some rock bassists make it their job to hold down the bottom of a song: to hone parts that crisply but unobtrusively stake out a harmonic and rhythmic foundation, that are felt as much as heard. Phil Lesh, a founding member of the Grateful Dead who died Friday at 84, wasn’t one of them. Instead, Lesh’s playing carried songs aloft.

In the telepathic tangle of the Grateful Dead’s arrangements — never played the same way twice — Lesh’s bass lines hopped and bubbled and constantly conversed with the guitars of Jerry Garcia and Bob Weir. His tone was rounded and unassertive while he eased his way into the counterpoint, almost as if he were thinking aloud. Lesh’s playing was essential to the Dead’s particular gravity-defying lilt, sharing a collective mode of rock momentum that was teasing and probing, never bluntly coercive.

Lesh wasn’t a rock-and-roller by training or inclination. His 2005 memoir, “Searching for the Sound,” notes that his first instruments were violin and trumpet, that he soaked up classical music and big-band jazz, that he studied music theory and composition and drew life-changing inspirations from John Coltrane and Charles Ives. He and Tom Constanten, the Dead’s early keyboardist, were the band’s avant-garde contingent, a key aspect of the Dead’s ever-evolving improvisational fusion.

For all their free-form interludes, the Dead’s songs had clear landmarks and structures — some of them far trickier than the band’s nimble performances would let on. Lesh could stick to a riff, as he dutifully did in the intro to “Touch of Grey,” the Dead’s only Top 10 (and only Top 40) single. But when the verse arrived, he was footloose again: nudging, scurrying, syncopating from below. His bass lines held hints of Bach, jazz, bluegrass, blues, Latin music and far more, as he sought out new interstices each time through a song.

Lesh’s approach was far from undisciplined; from the beginning, it was intentional.

“I wanted to play in a way that heightened the beats by omission, as it were, by playing around them, in a way that added harmonic motion,” he wrote in his memoir. “I wanted to play in a way that moved melodically but much more slowly than the lead melodies sung by the vocalists or played on guitar or keyboard. Contrast and complement: Each of us approached the music from a different direction, at angles to one another, like the spokes of a wheel.”

Garcia and Weir were the Dead’s main songwriters and singers; Lesh’s unassuming baritone generally helped fill out vocal harmonies. But he did contribute to staples of the band’s live sets. He collaborated on “Truckin’” and “St. Stephen” and provided the music of “The Eleven” (named for its tumbling 11-beat rhythm). Lesh wrote (with Robert Hunter) and sang lead on one of the Dead’s finest existential benedictions, “Box of Rain.”

Another Lesh song rightly beloved by Deadheads is “Unbroken Chain,” which he wrote with his friend Bobby Petersen. It begins in folky contemplation, but unfolds with musicianly surprises: unexpected meter shifts, jazzy interludes and shivery electronic tones. Lesh sings thoughtfully about solitude, natural beauty and the idea that “forgiveness is the key to every door.”

Lesh parted ways with the other surviving members of the Grateful Dead after their 50th-anniversary “Fare Thee Well” concerts in 2015. He didn’t rejoin them as Dead & Company continued to stage lucrative festivals and massive events like their 2024 residency at the Sphere in Las Vegas. But he kept making music, playing theater-size venues all the way into this summer with assorted lineups of Phil Lesh and Friends. He was still improvising through the Grateful Dead catalog, still trying new variations and reflexes, possibilities and whims. As he sang in “Unbroken Chain,” he was still “listening for the secret /searching for the sound.”