Question about Motzart's use of a C# in his Sonata k545

[bustthewave]

So I'm learning this piece, and just posted it up what i've done so far for feedback; and decided to post about this as well. I'm wanting to delve as far into composition as i can, and trying to understand the classical style... it's tough through self study.

So in Motzart's piano sonata k 545, it's in C major. The opening clearly defines the tonic, modulating occasionally between F and G, but still clearly in C major. Then the right hand runs begin. To me as i'm playing and listening to this piece, the tonality of it becomes quite undefined here, and it seems to be much more modal in nature than in a specific key.

The first run is essentially the A minor scale, followed by ascending runs down the keyboard to D. On this run, Motzart chose to sharp the tonic, C (which obviously works beautifully, and I think it also sounds like it belongs because the piece here has lost it's sense of a solid key, as it moves towards G major in the next little section). I hear the power and the intensity that the D minor scale creates here, but I'm wondering what the compositional principal is behind it?

The C sharp from what I can tell, doesn't lend itself to the immediate following modulation into G major, since C# is not a note in the G major scale (I'm speaking based on my level of understanding, if there is some principal I'm missing... please fill me in ), yet it still 'feels' like the absolute perfect note.

The C sharp also doesn't seem to be repeating anywhere else. The major recapitulation I've seen in the piece is in F major, so i'm not sure what the C# is moving towards. but again, I "feel" it's natural movement when i listen to the piece. Why is that?

Sorry if I'm talking about things way over my head, just trying to get some solid footing on what i've been studying.

[drkilroy]

I do not know very much about theory, but I think this is a part of the modulation to G major. The section after measure 11 is in G major and the run in measure 9 (about which you were talking about) is in D major, which is the dominant for G major. So, the run in D major leads to the G major section and that is how it works so well.

Best regards, Dr

[bustthewave]

Actually I didn't even consider that D was the dominant of G! I'm still trying to figure out exactly HOW classical composers modulated (and as i listen to more and more classical pieces, it definitely, to my ears, sounds like there endless possibilities. I need to get sheet music for concertos and symphonies and the like ).

But wait, wouldn't it be the D minor scale, since the F# is flatted? I have lots of random gaps in my knowledge of music theory, that i'm trying to fill in. Minors is one of them. Either way, I still think you were dead on... and listening to it yet again, I think it sounds so intense because prior to bar 9, it was a choleidoscope of keys, and there wasn't a firm grasp of a solid tonic. The D scale then maybe affirms that we are officially moving towards stability, and a new tonic, while also directly destroying our expectations of returning to the original tonic, by sharping that very note (the C).

Definitely not trying to speak with authority, just trying to work this out in my mind.

But geese mozart was some kind of brilliant. Even in this sonata he considered as a beginner piece...

[drkilroy]

But wait, wouldn't it be the D minor scale, since the F# is flatted?

Yes, you are right. But remember that if we are not using some modes (not something Mozart would do, I think), the dominant must be major. As you may see, in measure 10, just before the modulation to G major, there is F-sharp, which makes the D dominant major. So finally there is a D major chord which leads to G major section.

Even in this sonata he considered as a beginner piece...

It is not very easy, however.

Best regards, Dr