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.
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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:
^6 ^7 ^1are
6 6 (or
6 6/5 ), yielding
IV6 V6 Ior
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).
vi) is typically grouped with the tonic (
I), reflecting its function in a deceptive cadence. To prepare the dominant, one typically expects
IV. (See the Wikipedia articles on harmonic function and the submediant.)
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
In other words, the progression is defined by the bass motion: 8-7-6-7-8.