Deep Purple are an English rock band formed in Hertford in 1968. Along with Led Zeppelin and Black Sabbath, they are considered to be among the pioneers of heavy metal and modern hard rock, although some band members claimed that their music cannot be categorised as belonging to any one genre. They were once listed by the Guinness Book of World Records as "the loudest pop group", and have sold over 100┬ámillion albums worldwide, including 7.5 million certified units in the United States. Deep Purple were ranked #22 on VH1's Greatest Artists of Hard Rock programme. The band has gone through many line-up changes and an eight-year hiatus (1976Ô84). The 1968Ô76 line-ups are commonly labelled Mark I, II, III and IV. Their second and most commercially successful line-up featured Ian Gillan (vocals), Roger Glover (bass), Jon Lord (keyboards), Ian Paice (drums), and Ritchie Blackmore (guitar). This line-up was active from 1969 to 1973, and was revived from 1984 to 1989, and again in 1993, before the rift between Blackmore and other members became unbridgeable. The current line-up (including guitarist Steve Morse) has been much more stable, although Lord's retirement in 2002 has left Paice as the only original member never to have left the band. As of 2012, the band has not been inducted into the Rock and Roll Hall of Fame.