When did heavy metal start?

The first true, undisputed heavy-metal record was Black Sabbath's 1970 debut album, with its first track "Black Sabbath" the first undisputed heavy-metal song.

Many acts before this can claim to be psychedelic, heavy rock, heavy blues, hard rock or even proto-punk -- such as Iron Butterfly, the MC5, Led Zeppelin, Steppenwolf, The Stooges, even Jimi Hendrix. But no one denies that the first two Black Sabbath records were heavy metal, and that Black Sabbath was a heavy-metal band.