It was July 1954 when Elvis Presley was teamed with a couple of local Memphis musicians for a recording session at the Sun record studio. They had finished recording a cover version of a blues tune, "That's All Right". It was all right  but Elvis, Bill Black and Scotty Moore needed another song for the flip side of what was going to be their first release. They took a break to think it over but they didn't have much time because their studio time was almost over.

The three musicians did what most young musicians do when left alone with their instruments--they started goofing around. Bill Black was the oldest at 28. Elvis was the kid at 19. Bill started slapping his stand up bass and making fun of an eight year old bluegrass tune by Bill Monroe called, "Blue Moon of Kentucky". He even sang in a high nasal voice like Monroe. Scotty joined in on lead guitar and Elvis picked up his rhythm guitar and began singing. That's when studio owner and engineer Sam Phillips told them to start over and he began the tape rolling with a little extra echo added. The record was quickly pressed and delivered to local radio DJs in Memphis. It soon appeared on stations in Nashville and New Orleans and the career of the "Blue Moon Boys" and Elvis had begun.

Here are Both tunes. Bill Monroe (with Lester Flatt & Earl Scruggs in his band) and the Elvis version: