Deep Purples is an English rock band that was formed in 1968, they are considered to be among the pioneers of heavy metal and modern hard rock, although their musical approach has changed over the years. Deep Purple, Led Zeppelin, and Black Sabbath together 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 the 1975 Guinness Book of World Records as "the globe's loudest band" for a 1972 concert at London's Rainbow Theatre, and have sold over 100 million albums worldwide. Deep Purple has survived a seemingly endless series of lineup changes and a dramatic mid-career shift from grandiose progressive rock to ear-shattering heavy metal to emerge as a true institution of the British hard rock community. Their revolving-door roster launched the careers of performers including Ritchie Blackmore, David Coverdale, and Ian Gillan.