“IT WAS 20 years ago today”, Paul McCartney roared at the album’s opening, that “Sergeant Pepper taught the band to play.” Make that 70. Released in Britain on May 26th 1967, and in the United States a week after, “Sgt. Pepper’s Lonely Hearts Club Band” would become one of the most celebrated records of all time. Half a century on, however, Mr McCartney’s next line makes for a fitting epitaph. The album has indeed gone in and out of style.

“Sgt. Pepper” is one of a select group of albums to have sold more than 10m units in the United States, with 5m in Britain (the third-highest in the country’s history). Its cover, with the Fab Four sporting garish military dress in front of a wall of famous figures, is rivalled only by the zebra crossing on Abbey Road in the iconography of the world’s most famous band. Rolling Stone magazine has voted it the greatest album of all time.

Yet when rating Beatles songs, the same Rolling Stone included only two from “Pepper” in the top 50. On Spotify, not a single track from the album appears in the top 25 most played (see chart). A middling “Sgt. Pepper” track gets more attention than most, suggesting that some digital listeners still persevere with listening to albums. But “Here Comes the Sun” has been streamed nearly as often as all of “Sgt. Pepper”’s tracks taken together.

The importance of “Sgt. Pepper” will be lost on anybody who stumbles across it via the shuffle button. It marked a transformation, both for the world’s biggest band and for popular music. Following three years of Beatlemania and grinding tours, the band quit the road in 1966. That gave them time to tinker in the studio, making songs that they wouldn’t need to play live: they spent five months and £25,000 on “Sgt. Pepper”, having recorded their debut album for £400 in a single day. They changed their look, topping their cheeky grins with “university-days bad-idea facial hair”. The record came in a gatefold, book-like cover with printed lyrics, rather than the customary sleeve. Its music was richer and more experimental than anything they had produced before.

The band had been dabbling for a while. Bob Dylan had given them marijuana in 1964, and within two years they were writing lyrics about “Dr Robert”, a physician who could expand your mind with “a drink from his special cup”. George Martin, the band’s producer, first included a string quartet on “Yesterday” in 1965, the same year that George Harrison tried a sitar on “Norwegian Wood”. “Revolver”, released in 1966, had strings for “Eleanor Rigby”, brass for “Got to Get You Into My Life” and two Indian-instrumented songs. The record was the first to be mixed by Geoff Emerick, a 20-year-old sound engineer who sped tapes up, dubbed them on top of each other and played them backwards.

To find these elements in the next album was not surprising. But most tracks on “Sgt. Pepper” had them all, from the eerie organ introduction of “Lucy in the Sky with Diamonds” to the wailing orchestral crescendo of “A Day in the Life”. Impressed by the fullness of the Beach Boys’ music, with its eclectic rhythms, sounds and harmonies, Paul became determined to produce a sonic tapestry of his own. He became the creative force behind the album—a directing role that the others would come to resent—producing seven solo compositions to John’s three and recording his melodious bass parts separately. The tension was a creative one: this was the first album in which the Beatles consistently sang different words over or at each other, in harmonious discord.

Their lyrics had been maturing: “Sgt. Pepper” had none of the anodyne love songs that brought them fame. At times John neared his best, with the psychedelic poetry of “Lucy in the Sky with Diamonds” and the poignant car crash of “A Day in the Life”. Paul continued his fascination with everyday life, to the growing irritation of John, penning songs about a parking warden (“Lovely Rita”), an unhappy girl (“She’s Leaving Home”) and grandparents (“When I’m 64”). “Getting Better” had more edge: a deceptively dark Paul piece about an “angry young man” who failed at school and beats his wife, which sounded more like one of John’s.

Arguably the best lyrics of early 1967 didn’t make it onto the album. “Strawberry Fields Forever” and “Penny Lane” were John and Paul’s odes to Liverpudlian childhood: one an acerbic song of estrangement with a flute-like Mellotron, the other a collage of suburbia set to French horns. These were the hits that “Sgt. Pepper” never had, released instead as a single. Joe Cocker’s version of “With a Little Help from My Friends” rocked much harder than Ringo Starr’s, reaching number one in the British charts. That leaves the Beatles’ most celebrated album without a signature tune for the Spotify generation. The Edwardian-themed tinkling harpsichords and honking brass were novel in 1967 but are quaint in 2017: it is the acoustic numbers or the rockers that make it onto digital playlists. “A Day in the Life” is an undisputed masterpiece, though the overdubbed howl of a 40-person ensemble does not make for easy listening.

That, however, is the album’s real legacy. Though the Beatles were not the only group writing while high, messing about with tape decks and hiring orchestras, they were the most conspicuous. By putting out their first songs of more than four minutes and wrapping them (albeit loosely) in a story, they were pre-empting the concept albums of the early 1970s (see chart). Prog Archives, a website dedicated to progressive rock music, classifies the Beatles as “proto-prog”. Its users rate the lengthy and complicated work of later bands more highly, but the influence of “Sgt. Pepper” is undeniable. No experimental album of the period is mentioned more often on Wikipedia, for example. Yes, who formed a year later, started by playing Beatles covers; Pink Floyd were invited to watch the recording of “Lovely Rita”. They could doubtless confirm that the production was second to none.