Side (recording)

The terms A-side and B-side refer to the two sides of 78, 45, and 33 1/3 rpm phonograph records, whether singles, extended plays (EPs), or long-playing (LP) records. The A-side usually featured the recording that the artist, record producer, or the record company intended to receive the initial promotional effort and then receive radio airplay, hopefully, to become a "hit" record. The B-side (or "flip-side") is a secondary recording that has a history of its own: some artists released B-sides that were considered as strong as the A-side and became hits in their own right. Others took the opposite approach: producer Phil Spector was in the habit of filling B-sides with on-the-spot instrumentals that no one would confuse with the A-side. With this practice, Spector was assured that airplay was focused on the side he wanted to be the hit side.

Music recordings have moved away from records onto other formats such as CDs and digital downloads, which do not have "sides", but the terms are still used to describe the type of content, with B-side sometimes standing for "bonus" track.

The first sound recordings at the end of the 19th century were made on cylinder records, which had a single round surface capable of holding approximately two minutes of sound. Early shellac disc records records only had recordings on one side of the disc, with a similar capacity (both media could hold between three and four minutes by 1910). Double-sided recordings, with one song on each side, were introduced in Europe by Columbia Records in 1908 and by the late 1910s they had become the norm in both Europe and the United States; the ability to effectively double the amount of sound on the disc was one major factor in it rising to dominance over the obsolete cylinder in the 1910s. There were no record charts until the 1930s, and radio stations (by and large) did not play recorded music until the 1950s (when top 40 radio overtook full-service network radio). In this time, A-sides and B-sides existed, but neither side was considered more important; the "side" did not convey anything about the content of the record.

...
Wikipedia