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What makes a chord "stable"? What makes a chord "unstable"? Just saw this in my piano book, but it doesn't really explain what it means.

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    "Stable" isn't necessarily a strictly-defined musical term with a single meaning; it's more of a qualitative thing. It generally just means that the chord wants to move to some other chord for some reason. There are several things that might make a chord "unstable" in some sense (suspensions and inversions come to mind). Can you provide us with more context from the book, so we can figure out what it means in this case? – Caleb Hines Jun 29 '15 at 22:13
  • @CalebHines It was in a chart. It says that the tonic is the most stable chord in a key. The supertonic gravitates towards the dominant, etc. – Jacob Swanson Jun 29 '15 at 23:22
  • @SomeDudeOnTheInterwebs Of course. :) – Jacob Swanson Jun 29 '15 at 23:22
  • @SomeDudeOnTheInterwebs Ha! The only "horse note" I'm aware of is in Leroy Anderson's Sleigh Ride. – Caleb Hines Jun 29 '15 at 23:23
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    @Caleb Hines - I'm quite horse at the end of a night's singing. Anyone would think it's a foalish question. Maybe it's to do with that song Le Mare? Hay, I could go on... – Tim Jun 30 '15 at 6:45
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The concept of tonality is partially based on the idea that certain chords "want" to go to other chords. For instance, the dominant (V) wants to go to the tonic (I), mostly because it has the leading tone (scale degree 7). A more complex example is the augmented sixth chord. The augmented sixth is VERY unstable, because it has two notes (the flat-sixth and the sharp-fourth) that both want to move to the dominant (i.e. scale degree 5). You can hear this instability in the chord. So stability is primarily based on the chord's purpose in relation to the tonal structure you're in, but at the same time something like an aug-sixth is never going to feel stable.

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    However, put that aug.6 in a Blues song, call it a dom. 7, and it's o.k. – Tim Jun 30 '15 at 6:48
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    Dissonance also contributes to the sensation of insability. The tritone interval found in both the augmented sixth chord and the dominant seventh chord urgently wants resolving to a consonance – dennisdeems Jun 30 '15 at 14:49
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As mentioned in a comment by Caleb Hines, there is no clear-cut definition of the concept of stability in music. When talking about chords, the two notions of stability that I consider most important are the stability with respect to a given key, and the stability of a chord without any context.

In a given key, the tonic is perceived as the most stable chord, and the dominant is perceived as unstable because it needs to resolve to the tonic. The supertonic may be perceived to depart from the tonic, moving to the dominant, etc. However, outside of textbooks, this has to be taken with a grain of salt. If you analyze actual pieces you will find out that this concept of stability is very relative, and that chords don't always move in the most expected way.

Another concept of stability takes the chord out of its context, and only looks at its degree of consonance or dissonance. In jazz theory, any chord with a perfect fifth (i.e. also a dominant seventh chord) is considered a stable chord. Chords with a diminished fifth (e.g., a diminished or half-diminished seventh chord) are considered unstable. The practical consequence of this notion of stability is the fact that only stable chords can be preceded by their respective secondary dominant chord, because they convey some feeling of resolution.

  • +1. I really like that this answer distinguishes between the extrinsic stability of the chord within the context of the key (based on the scale degree of its root) vs. the intrinsic stability caused by the combination of consonances and dissonances within the chord, regardless of key. When I was considering answering this, that's the same direction I was going to take it. – Caleb Hines Jun 30 '15 at 14:45

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