Here’s an excerpt from Beethoven’s piano sonata no. 5, second movement, measures 13-16, with my analysis in blue (key of Ab major, 2/4 time):
My question is about the notes and chords in the blue box. At first I thought it was a Neapolitan or augmented sixth chord, but it’s not. I’m very curious about the augmented first degrees (A naturals) on relatively strong beats. Is this just chromatic material or is there something functional happening here?
The overall harmonic motion of the measure is from IV to ii6. The transition is made through the augmented triad rooted on 4 — in essence, the first A-natural is just an accented chromatic passing tone.1
However, the arrival on the ii chord is intensified by the presence of viio/ii (over the Db pedal tone), also in a relatively accented position.
1 This same chromatic motion between a major chord and its relative minor in first inversion forms the classic accompaniment of the song "Brazil" ("Aquarela do Brasil", Ary Barroso [1939]). I really like this progression and also mention it in How do I use augmented chords in my progressions?.
I agree that this is basically moving from IV to ii6. But instead of an augmented chord, I'd treat that A as a chromatic passing tone. If you look at the line, Beethoven didn't have any emphasis on that A. If he really wanted that to be a chord, he'd make it more apparent. An augmented chord is usually a rare case in the classical period based on how I was taught.
Haydn used it in one of his string quartets and that chord is made clear - strong beat + all notes of the chord are played.
