The Dark Side of the Moon is the eighth studio album by English progressive rock band Pink Floyd, released in March 1973. It built on ideas explored in the band's earlier recordings and live shows, but lacks the extended instrumental excursions that characterised their work following the departure in 1968 of founder member, principal composer and lyricist, Syd Barrett. The Dark Side of the Moon's themes include conflict, greed, the passage of time and mental illness, the latter partly inspired by Barrett's deteriorating mental state. The suite was developed during live performances and was premiered several months before studio recording began. The new material was recorded in two sessions in 1972 and 1973 at Abbey Road Studios in London. The group used some of the most advanced recording techniques of the time, including multitrack recording and tape loops. Analogue synthesisers were given prominence in several tracks, and a series of recorded interviews with the band's road crew and others provided the philosophical quotations used throughout. Engineer Alan Parsons was directly responsible for some of the most notable sonic aspects of the album, and the recruitment of non-lexical performer Clare Torry. The album's iconic sleeve features a prism that represents the band's stage lighting, the record's lyrical themes, and keyboardist Richard Wright's request for a "simple and bold" design. The Dark Side of the Moon was an immediate success, topping the Billboard Top LPs & Tapes chart for one week. It subsequently remained in the charts for 741 weeks from 1973 to 1988. With an estimated 50 million copies sold, it is Pink Floyd's most commercially successful album and one of the best-selling albums worldwide. It has twice been remastered and re-released, and has been covered in its entirety by several other acts. It spawned two singles, "Money" and "Time". In addition to its commercial success, The Dark Side of the Moon is one of Pink Floyd's most popular albums among fans and critics, and is frequently ranked as one of the greatest rock albums of all time.