How did the hand gesture “devil horns” get started at metal music concerts?
Well, the ‘devil horns’ as you put it is a gesture that has been used in cultures for centuries, it’s most famous use being to ward off evil spirits. Ronnie James Dio, of Rainbow, Black Sabbath and Dio fame is generally seem as the person to first introduce the gesture in metal music, and as he was so influential throughout the 70’s and after it was just accepted and used. I believe that it’s link with Satanism is somewhat convoluted, in the same way that Pagans (especially Wiccans in the US) are seen by many as Satanists. There is actually no evidence to suggest it has anything to do with Satan…
