What Makes A Single?
In music, a single is a song considered commercially viable enough by the artist and record company to be released separately from an album, usually featured on an album as well. For more information, see single.
For this reason, ringtones, specifically mastertones, which are classified as "singles" in the RIAA database in addition to being counted as songs in the yearly IFPI Digital Music Reports such as the Digital Music Report 2009, are considered singles for the purposes of compiling this list.
Read more about this topic: List Of Best-selling Singles In The United States