Pink Floyd are an English rock band that initially earned recognition for their psychedelic rock music, and, as they evolved, for their progressive rock music. They are known for philosophical lyrics, sonic experimentation, innovative cover art, and elaborate live shows. One of rock music's most successful acts, the group have sold an estimated 74.5 million albums in the United States alone.[LINK: http://www.riaa.com/goldandplatinumdata.php?table=tblTopArt] RIAA, retrieved 22 April 2006 Pink Floyd had moderate mainstream success and were one of the most popular bands in the London underground music scene in the late 1960s as a psychedelic band led by Syd Barrett; however, Barrett's erratic behaviour eventually forced his colleagues to replace him with guitarist and singer David Gilmour. After Barrett's departure, singer and bass player Roger Waters gradually became the dominant and driving force in the mid-1970s,