- a different version (e.g., instrumental, live, acoustic,remixed version orin another language/text) of the A-side or another track
- another song from the same album, which the record company does not want to release on its own
- a song not considered good enough for the album
- a song that had not yet been completed at the time of the album's release
- a song that was intended to be B-side after all
- in concept records: a song that doesn't fit into the story line. (1)
