Deep Purple are a British rock band formed in London in 1968. They are considered to be among the pioneers of heavy metal and modern hard rock, although their musical style has varied throughout their career. Originally formed as a psychedelic rock and progressive rock band, they shifted to a heavier sound with their 1970 album Deep Purple in Rock. Deep Purple have been referred to as being part of the "unholy trinity of British hard rock and heavy metal in the early to mid-'70s", alongside Led Zeppelin and Black Sabbath. Listed in the 1975 Guinness Book of World Records as "the globe's loudest band" for a 1972 concert at London's Rainbow Theatre, they have sold over 100 million records worldwide. Deep Purple have also generated several successful spinoff bands, including Rainbow, Whitesnake, and Gillan.