Consonance and dissonance

Play C and G together. The sound settles, as if it had nowhere to go. Now C and B. There is friction, a pull for one of the notes to move. Both are harmonic intervals, with the notes sounding at once, and still they give opposite sensations.

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The first is consonant; the second, dissonant. Lacerda defines both by the sensation: a consonant interval gives rest, because its notes fuse; a dissonant one gives movement, because they do not (Lacerda 1966, ch. XXXIV).

Stable, not pleasant

Blatter warns about the words. In everyday speech, "consonant" becomes a synonym for pleasant and "dissonant" for disagreeable. Not in theory: consonant means stable, dissonant means unstable (Blatter 2007, part 3, art. 1). A dissonance is neither a mistake nor an ugly sound; it is a sound that asks to go on. A great deal of music lives on creating dissonances and resolving them.

Consonances

The books divide consonances into two groups (Med 1996, ch. XVII).

The perfect consonances are the perfect unison, octave and fifth; Lacerda names only the fifth and the octave, and Med adds the fourth, which has a story of its own. In them the notes fuse so markedly that they almost sound as one.

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The imperfect consonances are the thirds and sixths, major and minor. They sound stable, but with a less complete fusion, and so with more colour.

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Med adds an observation that ties the two classes to inversion. Perfect consonances stay perfect when inverted, so he also calls them invariable. Imperfect ones change quality, the major third turning into a minor sixth, so he calls them variable.

The perfect fourth divides the books

No interval causes as much disagreement as the perfect fourth.

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Lacerda puts it in a place of its own, the mixed consonance, with features between perfect and imperfect (Lacerda 1966, ch. XXXIV). Med lists it among the perfect consonances, beside the fifth, and records that it has not always been considered consonant and that some still call it mixed (Med 1996, ch. XVII). Blatter goes the other way: historically, the perfect fourth was treated as unstable, a dissonance tending to resolve to the major third, as in the example above (Blatter 2007, part 3, art. 1).

One possible explanation for the disagreement is context. A fourth on its own sounds stable; but when it appears above the lowest note of a chord, music tends to treat the upper note as something that must come down, and that is the fourth Blatter sees resolving to the third.

Dissonances, and their degrees

Everything that is not a consonance is a dissonance: seconds, sevenths, and augmented and diminished intervals (Lacerda 1966, ch. XXXIV). But not every dissonance is equally strong. Med grades them (Med 1996, ch. XVII):

Degree	Intervals
neutral dissonances	augmented fourth, diminished fifth
soft dissonances	minor seventh, major second
harsh dissonances	major seventh, minor second

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Listen to the difference between the first measure, with the major second and minor seventh, and the second, with the minor second and major seventh. Med orders the intervals from most consonant to most dissonant: perfect unison, octave, fifth and fourth; major third, major sixth, minor third, minor sixth; augmented fourth and diminished fifth; minor seventh and major second; major seventh and, last of all, minor second.

He also notes that many augmented and diminished intervals sound dissonant only by name. An augmented second sounds like a minor third; spelled another way, it becomes a consonance. That is why Med calls them conditional dissonances.

Dissonance asks for resolution

In traditional harmony a dissonant interval does not stay put: it resolves to a consonant one, following the tendencies of its notes (Med 1996, ch. XVII). An augmented fourth opens out to a sixth; a diminished fifth closes in to a third.

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In the first measure, F–B opens to E–C: the notes move apart. In the second, B–F closes to C–E: the notes move together. Med points out that in popular and modern music a dissonance often resolves to another dissonance, or does not resolve at all.

What comes next

Consonance and dissonance are the raw material of harmony. Stacking thirds, which are imperfect consonances, leads to chords; and the way dissonances resolve from one chord to the next is what gives a progression its direction.