A-side and B-side

A-side and B-side are words used in music. They used to mean the two different sides of vinyl records and cassettes. Singles were released on these from the 1950s. The words now mean the types of song that are usually placed on each side of the record. The A-side is normally the featured song. This is the one that the record producer hopes will receive radio airplay and become a "hit". The B-side, or flipside, is a secondary song that often does not appear on the artist's LP. 
"CD singles", often have two songs, but CDs do not have sides..