The B-side reference article from the English Wikipedia on 24-Apr-2004
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B-side

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B-sides are songs released on the same record as a single to provide extra 'value for money'. These songs are normally songs which did not make it onto the album, either because they were not considered good enough or because they did not fit in with the overall sound of the album tracks, or remixes. Occasionally they are instrumental versions of the A-Side, known as versions. The term comes from when a vinyl record had only two songs, the "A-side" (the single) and the B-side.