I think it bears pointing out that this piece doesn't seem to have an A major "section" at all. Wouldn't we all agree that a three-sharp key signature with a pedal point on f-sharp and suggestions of C-sharp chords is more indicative of f-sharp minor than A major?
Incidentally, the only A major chord I can find in the middle, is in the four-sharp 'Vivo' tempo beginning at the rit into the return of the 'Grave,' and is not even heard as an indication of A major being the key, but it is the start of a bass phrase which quotes his first prelude in c-sharp minor. A-G#-C#(here D-flat). So that has to be heard as the beginning of something, rather than the end of something.
In this sort of analysis, you have to distinguish which notes make up the primary harmony and which notes are passing, neighbor, chromatic, etc...basically "filler" notes. You have to look for clues to discern this information.
That's true. Don't forget that most of our technique for analysing music comes from analysing Classical music. There are in this later music, not only passing notes, but harmonies which don't lead anywhere; coloristic harmonies that don't serve structural functions. The analysis we learn in conservatory is functional analysis, but it fails us when we try to analyse music with the richness of Rachmaninoff and many others.
In the 'meno mosso' for instance, there is really only one harmony, f-sharp minor. Everything else is a deviation from it, but structurally inconsequential. It is a meditative passage with more emphasis on poetic effect than harmonic plan, and should be analysed as such. Passages should be analysed not according to rules derived from one style of music, but
according to the way they were written.
- In the piu vivo section, I would focus on what is happing in the different line voicings. You are going to have a lot of passing/neighbor/chromatic action going on and I think you should ask yourself, "OK, where is this going?"
True. This question is also important: is it going anywhere at all? In the first bar under piu vivo, the chord is given very clearly: a dominant seventh on b-flat. For that bar, and the one following, that is the only harmony worth notating in Roman numerals. All the chromatic decoration of the upper syncopated quarters is more emotional than structural; the B-flat pedal point is the most important structural thing, along with the phrase that he repeats over and over: G-flat, F, B-flat, preparing the way for the eventual full quotation of his first prelude; the middle notes would seem to suggest a dominant seventh built on F, but I say they are mainly coloristic, and since it always returns to the B-flat, you don't need to notate every beat as if it was an independent harmony.
In the third bar, the pedal point remains, but the harmony moves up a notch in intensity, suggesting G-flat major, which comes rather easily from a dominant seventh on B_flat. The same principals apply for this 6/4 bar.
This is how I would approach analysing music of this sort. You have to think in much, much larger terms, harmonic movement over the span of bars, not just beat by beat or even bar by bar. it's not possible. Imagine analysing op.39 no.1 with every eighth note denoting a different harmony. It is enough to drive a person mad, and is sure not the way Rachmaninoff composed the piece.
My next point may seem insufferably pedantic, but I object to referring to a collection of bars in this sense as "sections." You mention the piu vivo "section," but there is no such section in musical terms; it leads into the Vivo without relenting in intensity, all the way to the return of the Grave and the culminative bass quotation of his first prelude, and the return of the first theme of this prelude. It may be more appropriate to designate the piu vivo all the way through to the Grave as one 'section.' maybe even the meno mosso. My point is: think larger. That is the only way to understand such music.
- When it looks like it switches into E at the Vivo section, consider that things could be respelled. It may help if you are trying to figure out the function of each chord. Look at the last measure before it switches back to Db at Grave. There is a huge G# (Ab?) pedal and you are going back into Db. Looks like a dominant function chord.
Think larger: there can't be a G-sharp pedal point because it only happens where you mention it for one bar; the same length of time as the whole-note A which precedes it. This is one phrase, the quotation of his first prelude.
- At the Grave section, a lot of the notes in the left hand runs might be passing tones. Again, use the melody to guide your harmonic thinking.
More appropriately the bass! In the piu vivo bars, the melody doesn't tell you anything about the harmony; it all comes from the bass, and the other parts' relation to it. The same here, with a tremendously long pedal point on D-flat. The harmony is essentially unchanging, Rachmaninoff has just colored it with chromaticisms and richness of ornamentation.
- At the poco piu vivo marking, the left hand runs turn into jumping chords with bass notes...those bass notes may hint at the underlying harmony that is covered with chromatic motion.
In this case, they don't just hint, but "hit" you over the head with it.

All the chromatics in the left hand are just decorations over the pedal point, and they always return to it, with unchanging rhythmic regularity!
I emphasize a more liberal approach when analysing music. It is too conservative to suggest that everything should be notated by Roman numerals, or to arbitrarily divide bars into sections, or to give equal significance to all parts, no matter how much or little they contribute. Music has to be analysed according to the way it was composed, not according to rules we learn from other styles.
Walter Ramsey