Heavy metal is usually based on riffs created with three main harmonic traits: modal scale progressions, tritone and chromatic progressions, and the use of pedal points. Traditional heavy metal tends to employ modal scales, in particular the Aeolian and Phrygian modes. Harmonically speaking, this means the genre typically incorporates modal chord progressions such as the Aeolian progressions I-VI-VII, I-VII-(VI), or I-VI-IV-VII and Phrygian progressions implying the relation between I and ♭II (I-♭II-I, I-♭II-III, or I-♭II-VII for example). Tense-sounding chromatic or tritone relationships are used in a number of metal chord progressions. The tritone, an interval spanning three whole tones—such as C and F#—was a forbidden dissonance in medieval ecclesiastical singing, which led monks to call it diabolus in musica—"the devil in music." Because of that original symbolic association, it came to be heard in Western cultural convention as "evil". Heavy metal has made extensive use of the tritone in guitar solos and riffs, such as in the beginning of "Black Sabbath". Heavy metal songs often make extensive use of pedal point as a harmonic basis. A pedal point is a sustained tone, typically in the bass range, during which at least one foreign (i.e., dissonant) harmony is sounded in the other parts.
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