There are two main features which I hope that you identified
as crucial moments in the form of the melody line of this section.
First is the way the harmony centres on the note C natural -
that is the dominant of F, in the central bars of the trio
from 52A after the double bar line, to bar 60.
The note C is repeated again and again as the music modulates,
and it's then harmonised by the chord C major...
..C minor...
..A flat major...
..then as a 6-4-5-3 suspension over an E flat major chord.
Here the music is modulated to A flat major,
but it immediately returns to the tonic F major
by reharmonising this same note, C natural,
with a dominant seventh chord on C.
So, this C natural is crucial to the structure of the melody
in these central bars.
The second feature which I think you may have picked out
is the note that comes straight after the section I've just analysed,
when the music returns to its tonic key, F major.
The melody settles firmly on A natural,
just as it did right at the beginning of the trio.
I think that this A natural
remains the most important note in the melody from here on.
I shall play it from the end of the modulation we have just analysed.
I think that the B flats which come at the end of this little section
reinforce A natural as the focus of the melody,
because they're neighbour notes
which sound as if they have to fall back to it.
I think this is why the final cadence,
which we analysed at the start of this video section,
is written the way that it is.
So, my analysis is that the A natural,
which is so prominent at the start of the section,
is taken up to a neighbouring note, B flat...
..then this is repeated...
..and then the main cadence has to emphasise the G natural
that leads between the A and the final F.
This is voice-leading.
The upper voice leads from A...
..through G...
..to F,
and this makes the moving around the tonic arpeggio in the final bars
sound just as a prolongation of this final tonic note in the melody.
So, how do we put these musical observations into a graph?
Let's build it up.
We start with the opening A natural,
and we can now say that this note connects
with its reappearance in bar 61 when the music returns to the tonic.
Then it moves to a neighbour note in bar 67,
and then moves through G to F at the main or structural cadence.
This line, prolonging A natural and then moving down to a final F,
is called the fundamental line of the piece.
Now, let's add more of the detail we've analysed.
The central section prolongs the note C natural as we have seen.
This is established at bar 53
and is reharmonised until it falls to a lower neighbour note, B flat.
Then it returns to C over an E natural in the base,
and it leads back to the A of the fundamental line.
I could have put in more detail
to show how this C natural moves through C minor and A flat major,
but this is middle ground detail,
and so I've concentrated on the way that the voices lead
through the whole of this little section,
from the key of C major in bar 53,
back to F major at bar 61.
Lastly, I'll add a little detail
to show how the first section moves up an octave to a top A
before descending to the C which will be the focus of the middle section.
And I'll show the arpeggiations at the beginning and the end.
I hope you've followed how I've constructed
this complete background graph,
which shows my opinion of how the structure of this trio makes sense.
To conclude this video section,
we're going to listen to the whole trio once more,
following the graph rather than the score.