Consonance, Dissonance and Interval Character

The idea

Intervals are not neutral distances. Each one has a character, and four families cover them all:

Family	Intervals	Character
Open consonances	P5, P8 (and P4)	Stable, hollow, powerful
Soft consonances	3rds and 6ths	Warm, sweet, emotional
Mild dissonances	M2, m7	Tension that colors without hurting
Sharp dissonances	m2, M7, tritone	Unstable, demanding resolution

• Open consonances are why the power chord works. It has no 3rd, so it does not commit to major or minor. Under distortion, the simple frequency ratio of the 5th (3:2) stays clean while 3rds turn to mud.
• Soft consonances are the sound of twin-guitar harmonies. Iron Maiden and Thin Lizzy lines are mostly parallel 3rds with the occasional 6th.
• Mild dissonances are the sound of sus chords and 7th chords: tension you can live in.
• Sharp dissonances are instability. The tritone is metal’s signature interval: the riff of “Black Sabbath” is built on it. The minor 2nd is the horror-film interval.

Dissonance is not bad. It is fuel. Music moves because tension resolves, and a piece made only of consonance is wallpaper.

Why distortion changes things. Distortion adds overtones to every note and also creates new tones from the sum and difference of the notes you play together. With a 5th or an octave those extra tones line up with the original notes, so the sound gets thicker. With a 3rd, and even more with a 2nd or a tritone, the extra tones land between the notes and the result is rough. That is physics, and it explains most of rock rhythm-guitar writing.

Listen to the families. Compare P5, M3, the tritone and m2 played together:

On your instrument

Play these pairs on guitar, first clean, then distorted, and listen to what the distortion does:

Pair	Where	What you should hear
Power chord (root and 5th)	6:5 and 5:7	Stays clear and gets bigger
Major 3rd	6:5 and 5:4	Clearly rougher under gain
Tritone	6:5 and 5:6	Unstable, evil, wants to move
Minor 2nd	2:4 (D♯) and the open 1st string (E)	A grinding beat between the notes

A tight minor 2nd is awkward to finger on guitar, which is why the open-string trick above is the easy way to get one. It is also why guitar voicings usually spread dissonant notes apart. More on that in Module 3.

On piano, play E and F together in the middle of the keyboard, then move the F up an octave. The same two notes become a minor 9th: still tense, but with more air. Distance softens dissonance.

Worked example

Two riffs on the same rhythm, straight 8th notes, palm-muted. Riff A uses only the root, 5th and octave. Riff B swaps two notes for a tritone (B♭) and a minor 2nd (F).

Riff A      1 & 2 & 3 & 4 &
D|----------------2-------2-|
A|--------------2-------2---|
E|----------0-0-----0-0-----|
            1 1 5 8 1 1 5 8

Riff B      1 & 2 & 3 & 4 &
D|------------------------2-|
A|--------------1-----------|
E|----------0-0---0-0-1-0---|
            1 1 TT 1 1 ♭2 1 8

Riff A is solid and neutral: it could belong to a punk song or a pop-rock verse. Riff B is immediately darker. Two notes changed, and the genre changed with them.

Exercises

1. Write a 4-bar riff on the low E string using only roots, 5ths and octaves. Then write a second riff that adds one tritone and one minor 2nd as passing tension. Record both and compare. Which one feels like metal, and why?
2. Harmonize a simple tune you know in parallel 3rds, on guitar with a second recorded track or on piano with both hands. Mark the spots where a 3rd sounds wrong and a 4th or 6th sounds better. You are discovering counterpoint by ear.
3. Play every interval from the first lesson as a two-note chord with distortion on. Sort them into three lists: clean, usable, mud. Keep the list. It is your personal guide to writing rhythm parts.

Common mistakes

• Treating dissonance as an error. If your riffs sound polite, they probably contain no sharp dissonance at all.
• Leaving dissonance hanging by accident. A tritone or minor 2nd creates an expectation. Resolve it by step, or leave it unresolved on purpose.
• Playing full chords with high gain and then lowering the gain to fix the mud. Remove the 3rd instead.