Stable/unstable scale degrees vs. consonance/dissonance

A Hooktheory user asked in an email: What is the difference between stable/unstable scale degrees and consonance/dissonance in music?

At Hooktheory, we sometimes talk about stable and unstable scale degrees over a chord. Scale degrees are said to be stable if they belong to the accompanying chord. For example, a V chord contains scale degrees 5, 7, and 2. Therefore, scale degrees 5, 7, and 2, are considered stable scale degrees, whereas 1, 3, 4, and 6, are considered unstable scale degrees. In music theory, stable scale degrees are sometimes referred to as chord tones, whereas unstable scale degrees are referred to as nonchord tones.

The concept of consonance and dissonance is related to, but slightly different from the concept of stable and unstable scale degrees. Broadly, consonance refers to a combination of tones that sounds “pleasant” to the ear, whereas dissonance refers to a combination of tones that sound “unpleasant.” Therefore, a chord can be considered consonant or dissonant even in the absence of melody. The vanilla chords: I, ii, iii, IV, V, and vi, are all typically considered to be consonant sounds.

A general rule of thumb is that if a chord sounds consonant (pleasant) on its own, then adding stable scale degrees to it will allow it to remain consonant. On the other hand, adding an unstable scale degree to a consonant chord may or may not cause it to have dissonance.

As an example, many people feel that adding scale degree 2 to a I chord preserves its consonance. However adding scale degree 4 to a iii chord adds dissonance. The difference between these two cases has to do with the relative spacing between the scale degrees 1 :left_right_arrow: 2, and 3 :left_right_arrow: 4.

Just as the notions of “pleasant” and “unpleasant” are subjective, so too are the notions of consonance and dissonance. Since we all hear music differently, we all have different versions of which sounds we like and don’t like. Furthermore, having a little bit of dissonance in your song can be a good thing; for example, using dissonance that then resolves to consonance creates a momentary tension in the music which is sometimes more desirable than having no dissonance at all.

Ultimately it is up to the songwriter to decide how much consonance and dissonance to add in their songs.

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Two things I would like to add to this post although the original sender may not see this:

  • By stacking thirds above the bass, music in general has evolved to accept more non-resolving tones as chord tunes; this is why we gradually have the dominant seventh chord, then non-dominant seventh chords, ninth chords, augmented sixths, “11th” and “13th” chords, actual eleventh and thirteenth chords, and finally pitch class sets. If ^4 over a V chord is stable and metrically strong, V7 would be a better choice.

  • Scale degrees ^1, ^3, ^5 are stable over the tonic chord, and hence define the key (this is true for the diatonic scale). For example. if in the analysis the melody gravitates towards ^2 most of the time, usually the correct key is a fourth below / a fifth above the current key.

@HertzDevil What’s the difference between “11th” and “13th” chords and actual eleventh and thirteenth chords?

When a chord contains a nonchord tone resulting from a suspension, but the resolution does not happen, the whole chord itself becomes embellished; if there is a seventh above the root and the fourth (from 4-3) or sixth (from 6-5) lies above the seventh, that chord would be called an apparent 11th or 13th chord.

Real 11th chords and 13th chords are formed by stacking thirds as usual.