Pink Floyd were an English rock band who in the 1960s earned recognition for their psychedelic and space rock music, and in the 1970s, as they evolved, for their progressive rock music. Pink Floyd's work is marked by philosophical lyrics, sonic experimentation, innovative album cover art, and elaborate live shows. One of rock music's most critically acclaimed and commercially successful acts, the group has sold over 200 million albums worldwide including 74.5 million certified units in the United States. Pink Floyd influenced progressive rock artists of the 1970s including Genesis and Yes, as well as contemporary artists such as Nine Inch Nails and Dream Theater. Pink Floyd were formed in 1965, soon after Syd Barrett joined The Tea Set, a group consisting of architecture students Nick Mason, Roger Waters, Richard Wright and Bob Klose. Klose left shortly after, but the group had moderate mainstream success and were a popular fixture on London's underground music scene. The erratic behaviour of Barrett prompted his colleagues to add guitarist and singer David Gilmour to their lineup. Following Barrett's departure, bass player and singer Roger Waters became the lyricist and dominant figure in the band, which thereafter achieved worldwide critical and commercial success with the concept albums The Dark Side of the Moon, Wish You Were Here, Animals, and rock opera The Wall.