Prelude No. 21 in Bb Major by Frédéric Chopin

This prelude follows the following structure (click over each section for details):

	Bb major Two phrases	First, eight measures long phrase (measures 1-8).
	Bb major Two phrases	Second, eight measures long phrase (measures 9-16).
	Gb major Two phrases	First, eight measures long phrase (measures 17-24). The Gb major key suddenly appears without preparation. Gb major is the VI degree of the Bb minor key:
	Gb major Two phrases	Second, eight measures long phrase (measures 25-32). Chopin repeats the same melody from the previous phrase, but this time harmonized with a Gb dominant chord. This chord is used as an augmented sixth chord of Bb major to return to the main key in the next section.
	Bb major One, 12 measure phrase.	Measures 33 a 45. These measures emphasize the dominant chord using numerous extensions. The prelude reaches its climax in measures 39 and 40. A cadence to the tonic chord prepares the coda.
	Bb major 12 measures coda.	Measures 45 to the end.