Heavy Metal as a distinct style was largely birthed by Black Sabbath in 1970. A defining characteristic of the genre is the use of the 'Tritone' or 'Augmented Fourth.' In medieval music theory, this interval was known as 'Diabolus in Musica' (The Devil in Music) because of its dissonant and unsettling sound. Black Sabbath guitarist Tony Iommi, who had lost the tips of two fingers in a factory accident, used light-gauge strings and tuned his guitar down to make playing easier. This lower tuning, combined with heavy distortion and the tritone interval, created the dark, 'heavy' sound that defines the genre. Metal also emphasizes technical proficiency, particularly in fast 'shred' guitar solos and complex drum patterns featuring double-bass pedals. The genre's aesthetic of power and rebellion is often reinforced by aggressive vocals, ranging from operatic belts to guttural growls, and high-volume amplification.