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 2, and 3 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.