A Hard Day's Night mystery - What is the opening chord to that song?

The opening chord to “A Hard Day’s Night” is one of the most memorable in rock history. In The Complete Beatles Recording Sessions, author Mark Lewisohn quotes Beatles’ producer George Martin as saying, “We knew it would open both the film and the soundtrack album, so we wanted a particularly strong and effective beginning. The strident guitar chord was the perfect match.” But what exactly was that mysterious chord? (hit the link for the full scoop)