On Friday, February 10th, 1967, the Beatles threw a party at EMI Studios on Abbey Road in northwest London. The occasion: the recording of twenty-four bars of improvised crescendo, played by a forty-piece orchestra, for "A Day in the Life," the climax of the band's then-in-progress masterpiece, Sgt. Pepper's Lonely Hearts Club Band. Special guests included Mick Jagger, Keith Richards, Donovan and the Monkees' Michael Nesmith. At the Beatles' request, the orchestra members wore formal evening dress with funny hats, clown noses, fake nipples and, in the case of the lead violinist, a gorilla's paw on his bowing hand. Engineers Geoff Emerick and Ken Townsend taped the musical chaos on a pair of linked four-track machines, making this the first-ever eight-track recording date in Britain. "It only took three quarters of an hour to get [the machines] in sync," Townsend says. "The hardest part was hauling them upstairs to the control room."

The entire evening produced only thirty seconds of music (used twice in the final song). But the session was typical of the flamboyance and nerve that John Lennon, Paul McCartney, George Harrison and Ringo Starr put into the creation of Sgt. Pepper. "We were fed up with being Beatles," McCartney has said, referring to the matching suits and screaming girls they left behind after retiring from live concerts, at the end of August 1966. "We were not boys, we were men. . . artists, not performers." Sgt. Pepper was the willfully extravagant proof, a landmark achievement in technicolor sound, unifying concept and songwriting ambition.

The Beatles recorded almost every note of Sgt. Pepper in one room, Abbey Road's humble, white-walled Studio Two. (The orchestral session for "A Day in the Life" was a rare exception, held in cavernous Studio One, typically reserved for symphonic dates.) Number 3 Abbey Road was built in 1830 as a lavish private residence, with nine bedrooms, servants' quarters and a wine cellar. By 1967, EMI's studios there were drab and aging, compared to the rapidly evolving needs of the Beatles' principal composers, Lennon and McCartney. "I would come up to new problems every day," producer George Martin recalled. "The songs in the early days were straightforward, and you couldn't play around with them too much. Here we were building sound pictures."

The Beatles found ecstasy in invention. A percussive effect in McCartney's "Lovely Rita" was official EMI toilet paper (printed with the words THE GRAMOPHONE COMPANY LTD) blown through a comb. Lennon wanted to use an authentic steam organ for his circus fantasy "Being for the Benefit of Mr. Kite" (inspired by a Victorian show poster he'd bought in an antiques shop). But Martin could not find one for the session, so he and the Beatles devised an otherworldly combination of harmonium, harmonicas (played by Harrison and Starr) and chopped-up tapes of a calliope. For his Eastern hymn "Within You Without You," Harrison turned Studio Two into a meditation room, playing sitar with a backing ensemble of Indian musicians, everyone seated on a carpet on the floor with lights dimmed and incense burning.

Released on June 1st, 1967, in a now-iconic gatefold cover by artist Peter Blake and photographer Michael Cooper, Sgt. Pepper immediately electrified the world. No other LP of rock's first half-century so richly defined its era -- the hope and the mutiny of the 1960s -- and completely redefined the outer limits of the recording experience. "It seemed obvious to us that peace, love and justice ought to happen," McCartney said. At the same time, "we recorded Sgt. Pepper to alter our egos, to free ourselves and have a lot of fun."