Explain to a pop pianist: GECE to GC#A#C# alberti bass transition in K.545 2nd movt

Question

As a pop pianist I'm trying to understand the logic in the alberti bass modulation that sounds beautiful and "right" but the notes seem way off the diatonic scale of G maj.

The key signature is G major. K.545 2nd Movt Bar 3 the alberti bass line is GECE GECE then GC#A#C# (with a C# in the RH melodic line).

My thought process would be: B♭ instead of A# makes more sense (borrowing G min?) and where does the C# come from (D♭ maybe as an GB♭D♭ looks like a diminished triad).

So why GA#C# instead of GB♭D♭?

The next bar reverts to G maj alberti base so is this a case of adding leading notes?

Answer 1

This is a case where it's probably helpful to think of the passage melodically rather than harmonically.

Consider each note in the Alberti bass as a separate voice in a melodic line. In that case, starting on m. 3 beat 2, we have

• E - C# - D in the top voice,
• C - A# - B in the middle voice, and
• G - G - G in the lowest voice.

Looked at this way, it's more clear that the C# and A# are serving leading-tone functions toward their respective notes. Notating with flats would suggest that the two pitches are "leading" downward rather than upward.

To analyze harmonically, the simplest explanation, IMO, is that the G-A#-C# chord is a "common-tone diminished chord" (G-Bb-Db), but spelled enharmonically to emphasize the upward movement of the resolution to G major.

Answer 2

Yes, 'adding leading notes' is a good way of thinking about it. Don't worry about excusing those notes by 'borrowing' or 'modal mixtures'. There's not even much point in trying to stick a chord-name label on them. He's just shifted all the upper notes down a semitone so he can bring them back up in the next bar. Think 'whammy bar' rather than 'harmony theory' :-)

Answer 3

You're focusing on that one bar, not on where it's going. If Mozart had followed that one bar with G-A-C (left hand), then the chromatic passing notes would have been written G-Bb-Db.

In other words: 1. it's a passing chord 2. its spelling is a function of where it passes to, not of any objective harmonic function.

Answer 4

This spelling makes sense even alone for these notes in fact preparing the G major in the next bar from below. Mozart could have remained on C major, but makes the whole thing more interesting. Then A# and C# would be the augemented 2nd and 4th of G major, which certainly is more at home in G major than a minor 3rd or a diminished 5th would be.

But we can even argue this spelling from theory: This chord can be seen as borrowing the dominant chord with small 9 of the associated mediant (b minor), which would be F#79b and provides the A# as leading tone into the B and the C# as regular fifth. You will see that this passage could very well resolve into B minor by having the bass resolve into an F# instead of remaining on the G.

The effect of this is of course subtle, as we do not get a strong cadence here, so this is perceived rather as a diminuition of the plagal cadence C - G (Mozart even has a parallel octave in the resolution between alto and melody).