side

The terms A-side and B-side refer to the two sides of 78, 45, and 331⁄3 rpm phonograph records, or cassettes, 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.

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