JS Bach: Well-Tempered Clavier, Book 1, Prelude #1 in C
This is a well-known and much studied piece, partly because it's deceptively simple.
The rhythm never changes: running eighth notes from start to finish, factoring out rhythm as an element. What remains is harmony.
The key is C (=red).
I've charted the first three measures as an intro, after which the piece lays out as 8 lines of 4 measures each. Each line ends in either a tonic or dominant chord. The performer slows at the last measures, so there appears to be an extra measure.
The intro establishes the key of C. The first two lines end on a G (=dominant) chord: in inversion at measure 4; in root position at m. The 3rd and 4th lines end on a tonic chord: in inversion at m. 12; in root position at m. 16.
Measures 17-20 follow a chromatic modulation, roughly analogous to the development of sonata form (or a song break). At m. 21 is the start of a long blue pedal point (=held bass note), which the harmonies float over. The pedal G finally resolves to tonic at m. 29.
I've been much influenced by the chart of this piece made by Heinrich Schenker.
(tempo slows slightly..)
Here's another way to look at the piece that brings out more of the specific decisions of the performer. In the following images, the amount of ink indicates the loudness of the note:
© 2003-9 N. Resnikoff