b-sides

The A-side and B-side are the two sides of phonograph records and cassettes, and the terms have often been printed on the labels of two-sided music recordings. The A-side usually features a recording that its artist, producer, or record company intends to receive the initial promotional effort and radio airplay and hopefully become a hit record. The B-side (or "flip-side") is a secondary recording that typically receives less attention; although some B-sides have been as successful as, or more so than, their A-sides.
Use of this language has largely declined in the early 21st century, given that the music industry has transitioned away from analog recordings towards digital formats, such as CDs, downloads and streaming, which do not have physical sides. Nevertheless, some artists and labels continue to employ the terms A-side and B-side metaphorically to describe the type of content a particular release features, with B-side sometimes representing a "bonus" track or other material.

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