The 45 RPM (revolutions per minute) record was introduced in 1949 by RCA Records as a competitive marketing move to rival the newly released Columbia Records Long Playing (LP) 12-inch disc that played at 33 1/3 RPM. It was originally sold in conjunction with a special record player that required a wide record spindle that fit the larger hole in the smaller 7-inch disc. Eventually the dueling formats wound up coexisting as the album and single, with turntables adapted to play both speeds and the original 78 RPM.
With the rise of rock 'n' roll, jukeboxes, and Top 40 radio in the 1950s, the 45 emerged as the dominant format for single play discs. The song on the "A" side was positioned to be a possible hit, while the "B" side (or flip side) was relegated to a lesser effort from the artist.
But radio stations didn't always follow the record companies' promotional push. On some occasions, the B-side wound up becoming the surprise hit. Sometimes both sides emerged as smashes, such as Elvis Presley's "Hound Dog"/"Don't Be Cruel" release in 1956. The Beatles were so popular, and their music in such demand, that in 1965 Capitol Records actually released "Daytripper" and "We Can Work It Out" as a double-A-sided single. According to recent Billboard statistics, Elvis and the Beatles are the most successful artists to achieve double-sided charted hits.