Naming a chord in the first movement of Mozart's K. 331?

Question

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

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.

Answer 1

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.

Answer 2

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 vii^o): Second measure
• vi-V6 (or vii^o): Third measure

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