6

How could one name the first chord in measure three in Mozart's A-major piano sonata K.331, first movement?

Mozart Sonata in A Major, K. 331, mm. 1-4

It looks like a F#-minor-7th-chord without the fifth, i.e. something like a substitution of the submediant VI for the tonic I.

3

Having thought about this for a while (and being unsatisfied with the lack of detail in my assertion that a D sounds more idiomatic than a C♯), I am posting an answer to elaborate.

My reasons for calling this chord a (modified) IV6 chord include:

  • I've always heard the F♯ to E interval as an unresolved 7-6 suspension.
  • Typical figures for a bass line ascending ^6 ^7 ^1 are 6 6 [5] (or 6 6/5 [5]), yielding IV6 V6 I or IV6 V6/5 I (see the example in Wikipedia's article on the rule of the octave, which harmonizes the ascending major scale as I V4/3 I6 II6/5 V IV6 V6/5 I).
  • The submediant (vi) is typically grouped with the tonic (I), reflecting its function in a deceptive cadence. To prepare the dominant, one typically expects ii and IV. (See the Wikipedia articles on harmonic function and the submediant.)
  • The first variation supports this interpretation; it has a D in the melody at this point.
  • A similar bass line appears in Mozart's variations on Ah, vous dirai-je maman, without the inner pedal, and it is invariably the bass of a IV6 chord.
2
  • This is great. When I can, I'll post a similar case for C#.
    – Aaron
    Jul 7 at 14:38
  • @Aaron I'll be curious to see what you come up with. I did find one bit of evidence that I neglected to cherry pick, that some might read as supporting C♯, but I don't think it's structural. The actual harmony in that case is, I believe, ii4/3.
    – phoog
    Jul 7 at 14:54
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It's F# minor without the fifth. The E is a pedal tone.

One way to establish this, is to play the opening measure without the E. The overall sense of harmony is unaffected, which helps demonstrate that the E is independent of the harmonic progression even though it is often a participant in it.

The core harmonic progression is

  • I: First measure
  • V65 (or viio): Second measure
  • vi-V6 (or viio): Third measure

In other words, the progression is defined by the bass motion: 8-7-6-7-8.

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  • I would call it a IV6 chord. That is, if I had to add a note there, I would add a D, not a C♯.
    – phoog
    Jul 6 at 12:55
  • Why do you think that?
    – Anna
    Jul 6 at 15:47
  • @Anna Are you asking why I think the E is a pedal tone; why I think it's a C# minor chord; both; or something else?
    – Aaron
    Jul 6 at 16:40
  • Aaron, I was referring to phoog. @phoog So again, why would you call it a IV6 chord?
    – Anna
    Jul 6 at 22:12
  • 1
    @Anna because a D sounds more idiomatic than a C♯.
    – phoog
    Jul 7 at 0:23

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