Though Pink Floyd's The Dark Side of the Moon is a concept album concerned with the ways that the pressures of modern society can lead to madness, its only song of social consciousness is "Us and Them," the lyrics for which address such issues as war and poverty in the stark terms of the late '60s and early '70s. The song began as a piano melody written by the group's keyboard player Rick Wright when Pink Floyd was composing music for the 1970 film Zabriskie Point, directed by Michelangelo Antonioni. The tune was intended for a scene depicting a confrontation between college students and police, but Antonioni rejected it. Later, when Pink Floyd was working on the album that would become The Dark Side of the Moon, they recalled the piece of music known as "the violent sequence," and Roger Waters wrote lyrics for it, retaining some of its mood from the film, as "Us and Them." David Gilmour sang the song, which had calm verses building to a crescendo on the choruses, with a heavy echo effect. The Dark Side of the Moon was released in the late winter of 1973 and became a massive commercial success, topping the American charts in April and eventually racking up sales of more than 15 million copies in the U.S. alone. Pink Floyd played "Us and Them" as part of the album's material in the early '70s, abandoned it for a while, then brought it back for 1977's In the Flesh tour to promote the Animals album. It was also featured by the reconstituted band on its 1987-1988 world tour and on the tour to promote the 1994 album The Division Bell, and a live version was released on Pulse (1995).