“The bassline and the vocal are two separate lines of research,” [Paul] says, pointing to the left and right sides of his head. “It’s quite an interesting thing, how you play when you’re singing live. Something funny happens to you, you have to split your whole being in two.”
Performing…’Fixing A Hole’ was especially challenging: “The bass is 3/4 or something, dunno what it is, and the vocal’s 4/4–he rotates his right hand above his head, his left around his stomach, to show the coordination problem. “You have to get the mathematics right in your head somehow. But suddenly it happens and it’s great.”
~~Paul McCartney, discussing playing bass and singing simultaneously, in Paul du Noyer’s book Conversations With McCartney.
In Their Own Words 