… Elvis Presley recorded “That’s All Right, Mama.” During a break in a recording session consisting mostly of slow ballads, Elvis, Scotty Moore and Bill Black began fooling around.
Sam recognized it right away. He was amazed that the boy even knew Arthur “Big Boy” Crudup — nothing in any of the songs he had tried so far gave any indication that he was drawn to this kind of music at all. But this was the sort of music that Sam had long ago wholeheartedly embraced, this was the sort of music of which he said, “This is where the soul of man never dies.” And the way the boy performed it, it came across with a freshness and an exuberance, it came across with the kind of clear-eyed, unabashed originality that Sam sought in all the music that he recorded — it was “different,” it was itself.
They worked on it. They worked hard on it, but without any of the laboriousness that had gone into the efforts to cut “I Love You Because.” Sam tried to get Scotty to cut down on the instrumental flourishes — “Simplify, simplify'” was the watchword. “If we wanted Chet Atkins,” said Sam good-humoredly, “we would have brought him up from Nashville and gotten him in the damn studio!” He was delighted with the rhythmic propulsion Bill Black brought to the sound. It was a slap beat and a tonal beat at the same time. He may not have been as good a bass player as his brother Johnny; in fact, Sam said, “Bill was one of the worst bass players in the world, technically, but, man, could he slap that thing!” And yet that wasn’t it either — it was the chemistry. There was Scotty, and there was Bill, and there was Elvis scared to death in the middle, “but sounding so fresh, because it was fresh to him.”
Peter Guralnick, Last Train to Memphis: The Rise of Elvis Presley