We began by studying the role of the different degrees of a scale, their use as part of a cadence, and the nature and role of the latter. We also looked into chord tones and non-chord tones in order to define which notes of a melody can be harmonized and which chords can be used to that end. In this quest for the perfect harmonization there are two supplementary tools that I will introduce in the following articles, namely chord stability (or instability) and harmonic rhythm.
A stable chord is one which doesn't produce any particular need to evolve towards another chord when listened to. Generally speaking, stable chords all have in common the fact that they do not include the fourth degree of the scale. In the major scale, we are talking about the I, I6, IM7, III, III-7, VI-, and VI-7 chords.
The first three are tonic chords....because their root note is the tonic, duh! The rest are called substitution chords. I'll explain what that's all about in a future article. For the time being, the only thing you need to know is that they are stable chords.
On the flip side we have unstable chords, which are often dominant chords. One of their characteristics is that they include the "tritone." The what? No, it's not an endangered species. In fact I already explained briefly what it is in the very first article of this series. Don't tell me you don't remember... The tritone is the interval composed of three adjacent whole tones, hence its name (although it's also called an augmented fourth).
This interval can be found on dominant seventh chords between the major third and the minor seventh. For instance, the G dominant 7th chord (G7, which is the dominant chord of the C major scale) is made up of the following notes: G, B, D, and F. The gap between B (which is a major third from the chord's root) and F (the minor seventh from the chord's root) is exactly three whole tones...i.e. a tritone.
Here's another example for those of you go are getting bored of the C major scale. The Eb dominant 7 chord (Eb7, dominant chord of the A flat major scale) is composed of Eb, G, Eb, and Db. In this case, the tritone is between G and Db.
If you play it on its own, you'll surely notice that it desperately pushes to be resolved, as you can hear in the following example:
It's precisely this tension that highlights even more the need of dominant chords to resolve to the tonic.
In an upcoming article we'll see other rules that determine the "attraction" of certain degrees and chords towards others. But here's a brief spoiler anyway: The IV degree of any given key produces instability, too, because it pulls towards the III degree, which is part of the tonic chords. Yet, the seventh of the dominant 7th chord is the fourth degree of the song's key.