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Vincent Persichetti's "Twentieth Century Harmony", at one point, states that a chord can be stable, even when dissonant. I'm curious as to whether the opposite can also be true.

I immediately think of a chord built with notes G, C, and E. This, to me, sounds consonant, as it is an inverted C major chord, yet it also wants to "resolve" to that same chord with a different inversion. However, common practice period ideology would say that the perfect fourth within the original chord is dissonant. I assume that this would make the entire chord considered dissonant.

Any thoughts?

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None of the terms 'dissonance', 'consonance', 'stable' or 'unstable' have precise and universally agreed-upon definitions in a musical context, so when people make statements like this, there's always the question of what they really mean. To me, consonance/dissonance is more about a momentary sensation of smoothness/roughness, while stability and instability are more a question of the listener's expectations - do they hear the harmony as 'wanting to go somewhere'?

a chord can be stable, even when dissonant

An obvious example of that might be the tonic seventh chord in blues.

I'm curious as to whether the opposite can also be true.

Perhaps something as simple as a major V triad in major tonality? It's consonant in and of itself, but with its leading note, very much wants to 'go somewhere' (to most listeners, at least).

I immediately think of a chord built with notes G, C, and E. This, to me, sounds consonant, as it is an inverted C major chord, yet it also wants to "resolve" to that same chord with a different inversion. However, common practice period ideology would say that the perfect fourth within the original chord is dissonant.

I think that's partly because that definition of 'dissonant' includes something of the notion of being 'unstable', although inversions of chords do also actually sound less consonant too, as the set of harmonics is further away from matching the harmonic series of the lowest note.

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    "An obvious example of that might be the tonic seventh chord in blues." "Perhaps something as simple as a dominant chord in major tonality?" Don't these contradict each other? In the first example, you are saying that the tonic seventh chord is dissonant. In the second, you are saying that a dominant seventh chord is consonant. Does the musical context change how you perceive the consonance/dissonance of a seventh chord? The reply is much appreciated, by the way. – Rory Dillon Jul 2 at 21:52
  • @RoryDillon For the second example I didn't mean the seventh, just the major dominant triad. I've edited the answer to remove the word 'dominant', which is unhelpfully overloaded. "Does the musical context change how you perceive the consonance/dissonance of a seventh chord" - by my definitions of consonance/dissonance, not so much, but of course consonance/dissonance are relative - a 'dissonant' seventh might be more consonant than surrounding, even more extended chords! Incidentally, timbre affects consonance and dissonance greatly. – topo Reinstate Monica Jul 2 at 23:05
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Given that you bring up a "stable, even when dissonant" chord, I'll use the definition of "dissonant" that involves more intervals in the chord that aren't octaves, fifths, thirds, or even fourths (i.e. involves higher-number ratios of note frequencies that aren't close to very low-number ratios like the perfect 5th's 3:2).

Yes, "the opposite can also be true". The major chord is a consonant chord by this definition of "dissonant", but major chords in the context of a tonal piece can include the Neapolitan (the major chord based on the ♭2 scale degree) and the major chord based on the ♯4 scale degree. Both are very unstable and sound alien in the context of the home key.

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Can a chord be consonant, but unstable?

Yes, as you yourself noticed, a C major chord in the 2nd inversion is clearly consonant but unstable.

However, common practice period ideology would say that the perfect fourth within the original chord is dissonant.

This is an error going back at least to Johann Joseph Fux in "The Study of Counterpoint" on page 20, and repeated by many theoreticians since. The trouble was that there was no distinction between dissonance and instability at that time.

Paul Hindemith in "Exercises in Two Part Writing - Vol. II" on page 21 notes that "The octave complement[s] [perfect 4th, inversion of the perfect 5th] ... show a plainly discernible instability as a result of the upper position of the root tone."

Historically, parallel 5ths and 4ths were frequently used in organum from roughly 900 A.D. to 1200 A.D., and considered consonant so your ear is in good company! ;-)

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I immediately think of a chord built with notes G, C, and E.

That particular choice is a bit of a problem, as you pointed out, because the fourth between G and C in old school counterpoint is dissonance.

But, I think you can make the case simply by using first inversion E C G. That voicing has no dissonant intervals. The intervals are: minor sixth, perfect fifth, and minor tenth. But, the chord is deemed unstable, because it is inverted.

First inversion, major/minor triads are consonant yet unstable chords.

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Stability also can depends on context. For an extreme example, check out Beethoven's piano concerto # 4, from about 0:56 to 1:10 here:

The piece is in G major, but Beethoven manages to make the root-position G-major chord unstable, which is pretty cool.

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