single

In music, a single is a type of release, typically a song recording of fewer tracks than an LP record or an album. This can be released for sale to the public in a variety of different formats. In most cases, a single is a song that is released separately from an album, although it usually also appears on an album. Typically, these are the songs from albums that are released separately for promotional uses such as digital download or commercial radio airplay and are expected to be the most popular. In other cases a recording released as a single may not appear on an album.
Despite being referred to as a single, in the era of music downloads, singles can include up to as many as three tracks. The biggest digital music distributor, iTunes Store, accepts as many as three tracks less than ten minutes each as a single. Any more than three tracks on a musical release or thirty minutes in total running time is an extended play (EP) or, if over six tracks long, an album.
Historically, when mainstream music was purchased via vinyl records, singles would be released double-sided. That is to say, they were released with an A-side and B-side, on which two singles would be released, one on each side. Moreover, traditionally, only the most popular songs from a previously released album would be released as a single.

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