In light of what would be released in its wake, the Beatles' Sgt. Pepper's Lonely Hearts Club Band is not the most fully realized concept rock album. But what little thematic structure it has began to take shape on Feb. 1, 1967, when they began recording the title track.

The session at EMI's Abbey Road Studio Two ran from 7PM until 2:30AM, with the Beatles doing nine takes -- two of which were complete -- on the basic rhythm track, with Paul McCartney and George Harrison on guitars. Ever willing to experiment with new recording techniques, and with a first-rate engineering staff at Abbey Road Studios able to make it happen, McCartney tried something different with his bass. Rather than sticking a microphone in front of his amplifier, a box was built to send the signal right into the board, a process that they called "direct injection," or DI.

"I think direct injection was probably used on Beatles sessions for the first time anywhere in the world," said Ken Townsend, an engineer in the technical department at Abbey Road, in Mark Lewisohn’s The Beatles Recording Sessions: The Official Abbey Road Studio Session Notes, 1962-1970. "We built our own transformer boxes [called DIT boxes] and plugged the guitars straight into the equipment."

This was Townsend's second major technical contribution to the Beatles' in the spell of less than a year. In April 1966, Townsend invented a process for automatically double-tracking (ADT) -- re-recording a sound milliseconds apart and placing them on a new track to give it greater depth -- which wound up being used on every lead vocal on Revolver. ADT was created to appease John Lennon, who was famously insecure about his singing voice and always looked for ways to change it.

Lennon was intrigued by what was being done to McCartney's bass, but wasn't entirely sure what it was. "John came up to the control room one day and asked if we could possibly inject his voice directly into the console," engineer Geoff Emerick said. Longtime Beatles producer "George [Martin] replied, 'Yes, if you go and have an operation. It means sticking a jack-plug into your neck!'"

The next day, the Beatles overdubbed their vocals to the master take. At this point, only "When I'm Sixty-Four" and "A Day in the Life" had been recorded (though the latter did not yet have its orchestral overdubs), and they hadn't figured out how to convey the idea that they were putting on the guise of a fictional band.

By the time the Beatles returned to the song a month later, the idea of it being a full-fledged concert by these mythical musicians came into play. On March 3, they brought in four French horn players to add some brass, which helped sell the idea that they were an old-timey band. They added the sound effects -- the tuning up the audience noise -- three days later.

Lewisohn also notes that the song doesn't end on Ringo Starr's first note of "With a Little Help From My Friends," but rather just before they sing "Billy Shears" in unison. That introduction had been a part of "Friends" from the first take on March 29. The reprise of "Sgt. Pepper," which segues perfectly into "A Day in the Life," was recorded in one session on April 1.

Beatles Albums Ranked Worst to Best