Deep Purple an English rock band formed in Hertford in 1968. Heavy metal and modern hard rock pioneers, although their musical approach changed over the years. Originally formed as a progressive rock band, the band shifted to a heavier sound in 1970. Deep Purple, together with Led Zeppelin and Black Sabbath, have been referred to as the "unholy trinity of British hard rock and heavy metal in the early to mid-seventies". They were listed In 1975 in the Guinness Book of World Records as "the globe's loudest band".  Deep Purple have sold over 100 million albums worldwide.