It’s hard to determine the actual moment, if there even is one, where “rock and roll” became “rock.” Rock ‘n’ roll, most specifically, is music born of blues via rhythm and blues, popularized by artists like Chuck Berry and Little Richard, who injected the fervor of jump blues (sometimes with a dollop of country) with a bawdy, stripped-down and youthful exuberance. That rollicking music was fun to dance to, and it took over youth culture in the mid-to-late 1950s, inspiring a generation of kids and teens. Rock feels like a much more eclectic term, one that describes music that’s sometimes bluesy, sometimes folky, sometimes pop; it can be as accessible and melodic as an Elton John ballad; or as dense and abstract as the most impenetrable David Bowie track. Rock is Funkadelic as much as it is Joni Mitchell as much as it is Radiohead.

And if there is an album that feels like the moment when “rock ‘n’ roll” became “rock,” the Beatles’ Revolver is that album.

Revolver stands as the band’s biggest leap, their most boldly progressive album and the album that reimagined rock circa 1966. Obviously, it’s long suffered from being in the shadow of the album that followed it, the revered Sgt. Pepper’s Lonely Hearts Club Band. Sgt. Pepper’s… was an album that was such an epochal moment in popular culture that it’s reputation has casually swallowed virtually every other Beatles album in hype—even those albums that represent entirely different periods within the Beatles canon and despite the fact that many of those other albums are perhaps more broadly influential.

And, as in the case of Revolver, even when those albums are superior to Sgt. Pepper’s… in virtually every measurable way.

But in and of itself, Revolver is quite an achievement, a testament to the collaborative power of what was, essentially, a simple four-piece rock band. The Beatles were always a collective—even when they were at their most frayed in the late ‘60s, the band’s chemistry shone through on their best recordings. And on Revolver, the complementary dynamic between John Lennon, Paul McCartney, George Harrison, and Ringo Starr reaches its creative zenith, buoyed by an eclectic set of songs and, in George Martin, a producer eager to realize his proteges’ ever-expanding creative vision. Sgt. Pepper’s sounds like a band deliberately, self-consciously attempting to craft a grand artistic statement—which makes it their most pompous and indulgent record. Revolver is the sound of a band brimming with curiosity, confident in its ideas and delivering uncalculated greatness. This was the Beatles eager to deconstruct “The Beatles.”

That isn’t to suggest that the Beatles’ first few albums don’t show a band committed to evolution—quite the contrary: Please Please Me is the raw document of an innocently energetic rock and roll group giddy to record its first album; With the Beatles highlights their myriad influences and the ever-growing palette with Lennon and McCartney as songwriters; the simple pop iridescence of A Hard Day’s Night belies a quartet fully in command of its own sound and voice. Many contemporary critics view Beatles for Sale as the band at best treading water or at worst regressing, but it reveals more evidence of a group increasingly under the influence of Bob Dylan—and it’s probably the first album where the stylistic divide between the respective songwriting of John Lennon and Paul McCartney first becomes apparent.

The year 1965 is when the Beatles began to truly outgrow the confines of Beatlemania in image and sound—all while still touring around the world, filming the laughably bad spy spoof Help! and attempting to appease their still rabid base of lovestruck teenagers. The album Help! is evidence of a band stuck between the teenybopper rock and roll that made it famous and a maturing folk-rock sound a la Simon and Garfunkel and The Byrds that it had helped to spawn. Rubber Soul would follow six months later, an album that discarded virtually all of the trademarks of their early Merseybeat sound, embraced emotional ambiguity over adolescent innocence and became the band’s first widely-hailed masterpiece—the start of the Beatles as “serious artistes.”

But from the moment you hear that fake count-in that opens the ornery “Taxman,” along with it’s jagged guitar solo (played by McCartney, not Harrison), punchy rhythm, thumping bass and off-kilter backbeat, it’s clear that the Beatles weren’t just far away from their Beatlemania sound, they were also uninterested in rehashing what they’d so successfully mastered on Rubber Soul. That album was barely six months old when Revolver was released, and two albums have rarely been so close chronologically but so disparate sonically and musically.

Aside from McCartney’s “For No One” and beloved acoustic pop ballad “Here There and Everywhere,” Revolver largely avoids the bittersweet love songs that dominate Rubber Soul; instead the lyrics are more abstract, topical and impenetrable—more indicators of Dylan’s influence on mid-’60s songwriting, but also less opaque than Dylan songs of the period. Lennon sang an ode to his dealer on the wry rocker “Dr. Robert,” going a step further than the band had up to that point in openly referencing drug culture; and “She Said She Said” is a spiraling snapshot of psychedelic pop—a druggy recollection of a morbid conversation with Peter Fonda that benefits from a loopy tempo change and some unexpectedly effective bass-playing from Harrison—not McCartney, who’d left the session after an argument with Lennon, reducing the band to a trio.

The use of the studio “as an instrument” is one of the most significant aspects of the legacy of Revolver and the Beatles. Artists had often attempted varying recording “tricks” in the studio, but in the mid-’60s, the Beatles began pushing themselves as a creative unit—and with such popular influence, it led to shifts in popular music. ADT recording was developed for the Beatles (specifically for Lennon, who loved dubbing his voice, but hated the tedium of manual double tracking) and first used by the band on Revolver. McCartney’s bass amplifications were so loud that EMI’s engineers at Abbey Road Studios were convinced the band would ruin studio equipment. Lennon sang into a suspended microphone—which was fed through a rotating Leslie speaker, while his bandmates and engineers were running tape loops physically around the studio—just to successfully create the multi-layered effects on songs like “Tomorrow Never Knows” with the limitations of 1960s recording technology.

Always limited in his compositions per album, Revolver is a showcase for George Harrison as an emerging songwriter. Harrison’s devotion to Indian music and spirituality earns scoffs from anyone keeping an eye out for appropriation and “exotic ethnic” fetishizing; but the “Quiet Beatle” immersed himself in a culture that resonated deeply with him—something that became even more important later but was obvious on Revolver. The influence of Indian musical patterns clearly affected the Beatles psychedelic work, and Harrison was the most committed to the artform. After his witless-but-effective sitar playing on the classic “Norwegian Wood” from Rubber Soul, Harrison delivered his first full foray into Indian music with “Love You To.” The song is “Beatles” only in name; Harrison is the only member of the band actually on the record; accompanied by Anil Bhagwat on tabla and other Indian musicians from the Asian Music Circle. “It was only when a Rolls-Royce came to pick me up that I realised I’d be playing on a Beatles session,” Bhagwat recalled later. “George told me what he wanted and I tuned the tabla with him. He suggested I play something in the Ravi Shankar style, 16-beats, though he agreed that I should improvise. Indian music is all improvisation.”

In addition to the aforementioned “Taxman,” George’s other contribution on Revolver is the dissonant “I Want To Tell You,” a song with lyrics that point to his growing spiritual perspective and uncertainty (“I feel hung up and I don’t know why”) and a sound set apart by a deliberately off-key piano that’s as jarring as the main guitar riff is infectious.

Lennon was becoming more and more insular in his writing. He’d always been the Beatles’ best lyricist, delivering the first glimpses of introspection in the band’s repertoire as far back as 1964. But he’d gotten darker and more conceptual on Revolver, songs like “I’m Only Sleeping” (with Harrison’s backwards guitar solos and processed, compressed production) operate on a literal and a figurative level—it’s a song that sounds like an ode to both laziness and consciousness (“Everybody seems to think I’m lazy / I don’t mind I think they’re crazy / Running everywhere at such a speed / ’Til they find there’s no need.”) And his sense of experimentation was arguably the boldest of the four; “Tomorrow Never Knows” is a song that there were no indicators for, the sound of four artists successfully creating something that had almost no parent in Western pop music. McCartney’s tape loops (and his bassline, on top of Starr’s drumming) turn the song into something transcendent and rave-like, but it’s undeniably a product of Lennon’s increasingly altered state and, in the case of the lyrics, his fixation on the Tibetan Book of the Dead and Timothy Leary.

Driven by a growing fixation with the Beach Boys, McCartney’s Revolver songs aren’t as sonically outside-the-box as Lennon’s or as lyrically ambitious as Harrison’s, but the Beatles’ purest musician delivered a set of songs that don’t sound like they belong on any album being created by a “rock and roll” band that was still known for playing Little Richard covers onstage. “Eleanor Rigby” is as striking and unusual as “Tomorrow Never Knows,” albeit in a completely different context. A melancholy look at lonely and elderly characters Eleanor and Father MacKenzie, the song features no guitars, bass or drums; just those piercing Psycho-esque strings against McCartney’s wailing, overdubbed harmonies. “Eleanor Rigby” is an odd song to shoot to No. 1 the same year that “Barbara Ann” and “Cherry Cherry” were smash hits. Inspired by the Lovin’ Spoonful and his fetish for vaudevillian cheeriness, “Good Day Sunshine” is one of the most unapologetically happy songs McCartney ever recorded (which says a lot) and, conversely, the elegiac “For No One” is McCartney at his most somber, an unsentimental recognition of loss that acknowledges heartbreak and ego; inspired by his dying relationship with actress Jane Asher. These were all songs that defy the conventions of what “rock” was supposed to be, and in doing so, set a standard for what “rock” could become. Lennon was inspired by acid and Harrison by spirituality; McCartney’s greatest muse was always music itself.

In regards to the emergence of psychedelic rock, Revolver isn’t the first album that signified the genre’s arrival. Love, the legendary Los Angeles-based band led by Arthur Lee, released their debut album three months prior to the Beatles releasing the “Paperback Writer/Rain” single that would “announce” them as a psychedelic band. And the 13th Floor Elevators were advertised as a psychedelic band around Texas in early 1966, with their first single, “You’re Gonna Miss Me,” released nationally around the same time as “Paperback Writer.”

But what the Beatles did was redefine the sonic template of what “psychedelic” would mean in rock music. Most early psychedelic rock is Dylan-esque wordplay heavily connected to the sound of ’60s garage bands, the drugginess is mostly associated with lyrical references and some quirky instrumentation. On Revolver, the Beatles don’t sound like garage rock and the drugginess isn’t just in the lyrics—this is music that oftentimes sounds like acid.

Revolver is famously notable for being the first Beatles album to bear the marks of the band’s increased immersion into LSD. Particularly, Lennon and Harrison had been taking acid since 1965, when both were given the drug by their dentist while at a dinner party. When they were recording Revolver, it had become a regular part of their lives—and Ringo was often taking it, as well. McCartney, on the other hand, had avoided LSD—at the time of Revolver’s recording and release, he’d yet to take the drug. “I really was frightened of that kind of stuff,” Paul would say during the Anthology documentary decades later. “It’s what you’re taught when you’re young: ‘Hey, watch out for them devil drugs!’ So when acid came around, we’d heard that you’re never the same; it alters your life and you’ll never think the same again. I think John was rather excited by that prospect—I was rather frightened by that prospect. Just what I need—to have some funny little thing where I can never get back ‘home’ again. Geez. Might not be the greatest move. So I delayed and I was seen to sort of stall within the group. There was a lot of peer pressure—I mean, talk about peer pressure—the Beatles?”

The album was predated by several weeks by “Paperback Writer” and the “Rain” B-side—releasing non-album singles was a common practice for a ’60s pop artist in the U.K. That single revealed that the Beatles were going in a new direction: “Paperback Writer” is one of the most aggressive guitar songs any rocker had recorded up to that point, complete with odd backing vocals and psychedelic harmonies (the new remix on Beatles 1 is a revelation). And “Rain” is another boldly innovative moment from the band—their first undeniably druggy song, complete with McCartney’s best bass performance as a Beatle, Ringo Starr’s most uncharacteristically sophisticated drumming, and Lennon’s winding, backwards ad-libs at the end. The Beatles had a lot of hits before and would notch a lot of hits after, but 1966 was definitely their boldest year, creatively.

But the benefit of being the Beatles is another interesting facet of Revolver’s impact and legacy. They were the biggest act in music throughout the 1960s and had rattled off 13 No. 1 hits by the time “Paperback Writer” arrived in the late spring of 1966. They were emboldened by their own creativity, but also by a commercial success that had given them tremendous confidence—and significant leeway. Otis Redding’s face still wasn’t on his album covers in 1965 and in late 1966, the Temptations were recording the blatant crossover album In A Mellow Mood, stark reminders of the two worlds in which black and white artists found themselves operating. Black artists forever had to be reminded that they were black in a business that preferred whiteness; encouraged to hide or alter their images and sounds significantly in order to appease the racism of both the record-buying public and the record industry itself. Jimi Hendrix would emerge in the U.K. a few months after Revolver’s release and his classic debut Are You Experienced? would arrive in the spring of 1967, heralding him as the new standard in rock guitar and an artist capable reshaping the course of the genre. But Hendrix also only scored one major chart hit (1968s era-defining “All Along the Watchtower”) and Rolling Stone’s Jon Landau called Are You Experienced "…unrelentingly violent, and lyrically, inartistically violent at that.” It seems likely that the Impressions would have been dismissed had they released something as childishly bizarre as “Yellow Submarine” as a single in the mid-’60s.

Revolver turning 50 may be a bigger deal for Baby Boomers who worship at the altar of the Fab Four than it is for anyone else. The Beatles are becoming increasingly passé in many circles, an act that is hyper-mythologized and grossly mass-marketed, both damnable offenses in the court of contemporary cool. And McCartney has become a man all-too-eager to revel in the nostalgia, forever showing up at high-profile events to sing those golden oldies—basking in the glow of the aging Woodstock generation’s adoration. But the Beatles’ contributions to music are still significant and that music still merits revisiting—even if it doesn’t quite spark the same kind of “Can you believe this?” it may have in 1966. The Beatles tearing down certain “conventional” standards in 1966 would inspire artists ranging from Hendrix to George Clinton in the years following the album’s release and despite Sgt. Pepper’s Lonely Hearts Club Band, it’s the best example of why the Beatles are heralded as innovators decades later. But Revolver is still an all-over-the-place masterpiece, the sound of four creatives in full bloom. It rewrote the rules and popular music never looked back.

It is being. It is being.