The ametrical opening chord of the Beatles' "A Hard Day's Night" is the most beguiling in the band's history. People have been trying to figure it out and/or reproduce it for more than five decades.
However, the recently posted video below might help do exactly that. It features the actual multi-track recordings from the band's 1964 sessions; you can hear what each Beatle—and producer George Martin—is playing on the track.
"We knew it would open both the film and the soundtrack LP, so we wanted a particularly strong and effective beginning," Martin said. "The strident guitar chord was the perfect launch."
It is, in fact, something akin to a G seventh suspended fourth (That, or a G7sus4/A, are considered the best way to reproduce the chord on a single guitar).
The mystery is caused by the fact that Martin is playing a piano chord atop Harrison's Fadd9 (or "F with a G on top," as he said in early 2001) played on his 12-string Rickenbacker, Lennon's Fadd9 played on his Gibson J-160E and McCartney's single note (D) played on his Hofner 500/1 bass.
George Harrison's 12-string Rickenbacker guitar solo was doubled on piano by Martin but tracked at half speed and sped up during mixing. This is why the solo from the studio version of "A Hard Day's Night" was ever-so-unsubtly edited into the otherwise-live version of the song on the Beatles' Live at the BBC album—and why it never sounds quite right on other live versions.