Deep Purple are an English rock band formed in Hertford in 1968, being pioneers of heavy metal and modern hard rock, having been a progressive rock band, which changed to a heavier sound in 1970. Deep Purple, Led Zeppelin and Black Sabbath, have been described as the "unholy trinity of British hard rock and heavy metal in the early to mid-seventies". In the 1975 Guinness Book of World Records they were given as "the globe's loudest band" for their 1972 concert at London's Rainbow Theatre, having sold over 100 million albums worldwide.