The Prelude 9 in E major has twelve measures, which are divided into three sections with clear cadential points. These three sections are A—from measure 1 to measure 5, B—from measure 5 to measure 9, and C—from measure 9 to measure 12.The tonality varies significantly through these three sections. In the first part A, the tonal center is basically built up on the tonic—E major with a slight embellishment of B major towards its cadential point from measure 3 to 4. This short period of B major serves the secondary dominant function of E major leading to the I of E major to conclude this section with an IAC at measure 5.

The following section, which is considered to be a developmental passage, has a very rich chromatic harmony color. The tonal area departs from E major to its N6 (bII)—F major, borrowing a IV from E on the downbeat of measure, traveling to f minor, leading to G# major—the chord enharmonizing Ab (measure 8). It eventually returns to E major at the end of measure 8 and concludes this sections with an IAC at measure 9.

The last section is back to E major, where the original theme is stated shortly before it is shifted to a different tonal area, F major. The tonality keeps changing at the following measure, in which two chords ofIII of E major are presented. The whole piece then is closed by a PAC carried by the V-I of E major at measure 11 and 12. The table below shows the shifts of tonality in this piece.
 

Measure#	A (mm.1-5)	B (mm.5-9)	C (mm.9-12)
Tonality	E-B-E	E-e-F-E-f-G# (Ab)-E	E-F-g-G-E
Comments	·Tonic is established through the chords of E major ·A short period of B major is used as a secondary dominant of E major.	·Departing from E major,III andVI appear to establish a third relations in the roots via modal mixture ·Short chord progressions on F major suggest the modulating to the Neoplitan 6 of E major.  ·This F major does not last long. It is taken to its parallel minor, f on the third beat of measure 7 and soon led to the relative major of f minor-Ab chord on the fourth beat of this measure. ·The Ab chord progressions are the enharmonization of G#, which is theIII of E major.	·Staring from E major tonic chord progression, a modal mixture appears when iv of E major is used to take the following chords to F major (N6).  ·The N6 chord progression borrows a chord from its minor ii (a g minor chord)at the down beat of measure 11(modal mixture again) and soon shifts to a G major ( IIIof E), which happens before at measure 5. ·V of E major then takes over this passage and takes this piece to a conclusion wherea PAC is presented.

A harmonic relationship of thirds is considered a significant motive through this composition. The function of third relations tends to lead to chords, which are quite foreign to the tonality. Examples found in the passages of usingIII, andVI to carry a bass motion of B-G-E occurring at measure 5 and 6 subtly demonstrate the idea of tonality expansion by means of the technique ofthird relations.The use of Neopolitan is the other prominent harmonic motive that is used by Chopin frequently in this piece. The chord progression based on F major occurring twice in this piece (m.6 and m.10) is an example.

Although this prelude in “largo” only consists of twelve measures, its colorful harmonic vocabularies such as compact harmonic rhythms, non-chord tones, substations, enharmonizations, modal mixtures, and chromatic relations of third provide a meticulous structure within which functions of expository, developmental, and terminative are tightly placed. It’s an example of typical Chopin, and of course, would be the textbook for chromatic harmonic progression.