Tony Iommi

The tritone and diabolus in musica

The tritone is an interval of exactly six semitones, directly splitting the octave in half. It is the most dissonant interval available and was historically called diabolus in musica, the devil in music. Medieval composers avoided it. Iommi built riffs around it.

The Black Sabbath riff (the opening of the self-titled song) is three notes: E, G, Bb. That Bb is the tritone above E. Over an E power chord, the Bb creates maximum tension. There is no resolution. The riff does not resolve the tritone, it sits on it. That decision to leave the dissonance unresolved is what defined the sound of metal.

Minor pentatonic and blues scale

Iommi's soloing vocabulary is rooted in the minor pentatonic and blues scale, the same foundation as Cream-era Clapton and Page. He grew up listening to British blues. The blues scale's flat 5 was particularly natural for him because it overlaps with the tritone he used in his riffs. The same dissonant note that defined his rhythm playing also appeared in his solos.

Phrygian mode for maximum darkness

Several of Iommi's most iconic riffs use Phrygian mode. The flat 2nd in Phrygian adds a layer of darkness and tension that natural minor does not have. Iron Man and War Pigs both draw from Phrygian elements. The half-step movement from bII down to the root is one of the most common metal cadences and Iommi established it before most metal guitarists were aware it was a mode.

Downtuned strings and the physical context

Iommi lost the tips of two fingers on his right hand in a factory accident before Black Sabbath formed. He had to retune his guitar down to relieve string tension so he could play. Black Sabbath typically used C# standard or lower. That downtuning did two things: it made the guitar physically playable for him, and it created a lower, heavier sound that became a defining characteristic of heavy metal.

Power chords in downtuned guitars became one of the core building blocks of metal because of this. The lower register gives riffs physical weight. Iommi's constraints shaped the aesthetics of an entire genre.

Riff construction over chord tone soloing

Iommi's greatness is primarily as a riff writer, not a soloist. His riffs are rhythmically precise, harmonically dark, and melodically memorable. He constructed them from short, repeatable motifs that work as both rhythm and melody simultaneously.

When he does solo, his approach is economical: short phrases, space, and a blues vocabulary filtered through the darker harmonic world of the rhythm parts. He does not try to demonstrate technique. He tries to match the emotional weight of the riffs.