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A bit confused at the moment regarding when a perfect fourth is classed as a consonance, and when it is classed as a dissonance. I have read here on Stack Exchange, that perfect fourth intervals are consonant when they are inverted perfect 5ths, unless they involve the bass- (eg 64-53).

Essentially, it is my understanding that:

  1. If a fourth is created by two chord notes in the upper voices, this can be treated as a consonance, and therefore does not need to resolve.

  2. If the fourth is created by two upper voices that are not both chord notes, then this requires resolution.

  3. If the bass is one of the voices, the fourth requires resolution even if both voices are chord notes.

In my example, bar 22 is Dm7 (ii7) in third inversion, followed by G (V) in first inversion in bar 23. The parts concerned are alto and tenor (alto clef). I would like to put a G (anticipation- highlighted) in the alto part. This non chord note forms a dissonance of a second with the F in the tenor part below. In the next bar, the two voices form a 4th, D-G (two chord notes of the GM chord). Is this fourth classed as a consonance, because it is an inverted perfect 5th in the upper voices? Or is it a dissonance, meaning that putting in a second interval before it is problematic?

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I think you've got it right. The fourth is a consonance if it's in the upper part of a chord (like E-G-C a C63). It's a dissonance when it's not part of the chord (as any other note would be.) It's a dissonance against the bass as this (at least since tonality became common) implies a 6-4 chord like G-E-C-G which is used in a few idiomatic places.

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In the next bar, the two voices form a 4th, D-G (two chord notes of the GM chord). Is this fourth classed as a consonance...

It is a dissonant non-chord tone in a suspension. The three parts of a suspension are (P) preparation, (S) supension, and (R) resolution...

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Basically the dissonance of the fourth is in relation to the bass. That is why the fourth in a first inversion 6/3 chord isn't treated as a dissonance. Relative to the bass the intervals of the upper voices are a third and a sixth.

But I think you should also consider how to resolve, because fourths get different handling in different contexts.

You want to be aware of the four 6/4 chord types - cadential, pedal, passing, and arpeggiated - and their handling.

The cadential and the pedal are the given a contrapuntal treatment similar to a suspension. There will be a fourth and sixth above a bass and those two tones resolve by stepping down to a third and fifth. Passing and arpeggiated move differently and involve movement not of the upper voices, but of the bass. In passing types the bass moves between the root and third of a chord like I V6/4 I6 or I6 V4/6 I and in arpeggiated types the bass just steps through the various chord tones like I I6 I6/4 I. You can find scores illustrating these in a good harmony textbook.

In all of those the fourth's dissonance is in regard to the bass and the different resolutions depends on the relative motion of the voices.

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  • Hi Michael. Yes, I'm aware of the different types of 6 4 chord, and thanks for the detail. My question actually refers to the fourth created by the tenor (alto clef) and alto parts in bar 23 (D-G). These two notes are both chord notes, and neither are in the bass. Would you class this as a consonance ? – EdB123 May 9 at 13:23

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