From Scientific Blogging: Beatles Unknown "Hard Day’s Night" Chord Mystery Solved Using Fourier Transform.

It’s the most famous chord in rock ‘n’ roll, an instantly recognizable twang rolling through the open strings on George Harrison’s 12-string Rickenbacker. It evokes a Pavlovian response from music fans as they sing along to the refrain that follows:

"It’s been a hard day’s night
And I’ve been working like a dog"

The opening chord to "A Hard Day’s Night" is also famous because, for 40 years, no one quite knew exactly what chord Harrison was playing.

There were theories aplenty and musicians, scholars and amateur guitar players all gave it a try, but it took a Dalhousie mathematician to figure out the exact formula.[continue].