By Troy Stetina
Guitar School, January 1992
Updated and rewritten (c) 2020 Troy Stetina
During the 16th century an elaborate system called musica ficta (falsified music) was created in which dissonances (notes that clash) and other "crimes against the ear" were avoided by altering the offending notes a half-step. The early church even referred to the extremely dissonant tritone interval as the 'diabolus in musica' (literally, the devil in music). The tritone's ugly turbulence was seen as the incarnation of evil, representing the seething chaos lying beneath our mortal reality!
But times have changed. After five centuries of musical evolution, these "crimes against the ear" sound no more than fairly interesting to the modern ear. Dissonance is no longer relegated to the confines of the underworld; today, it's an accepted musical tool to create harmonic tension. And because dissonance creates tension, it's not at all surprising that it's found at the core of today's most tense and aggressive music.
What is Dissonance?
Dissonance is the opposite of consonance (notes that play nice together). Figure 1, below, demonstrates a perfect consonance, then, at the other end of the spectrum, a strong dissonance. In this case, it's a tritone. Listen for its turbulent unsettledness. Distortion (gain) on the guitar tone will accentuate this aspect of harmonic turbulence even more. Figure 2 displays all the intervals within an octave (the so-called 'simple' intervals). I've indicated which ones are dissonances and which ones are consonances.
Two Kinds of Dissonance
Dissonances can reveal themselves in two different ways: harmonically or melodically. If dissonant notes ring together, it's called harmonic dissonance. That's what you heard in Figure 1. But if you play the notes in sequence, one after another, it creates a melodic dissonance. In this case, there isn't any turbulent clashing of notes. Instead, you hear an odd or disjointed-sounding leap between the notes. You'll often find this device lurking within the more interesting riffs you meet.
Figure 3, from "All Things Repulsive," and Figure 4, from "Bug Guts," are both excerpted from my book/audio method Thrash Guitar. These examples demonstrate each type of dissonance in action. I've placed an asterisk with either an "M" or an "H" to denote instances of melodic or harmonic dissonance, respectively. Figure 3 is loaded with that old "diabolus in musica," the tritone (or diminished fifth). Figure 4 uses both melodic and harmonic dissonances.