The prelude and the fugue of BWV 861 are set in G minor. Although no modulations are present, there are multiple times that the key deviates to the dominant.
Prelude The first two measures of the prelude are an elaboration of tonic. There is a chain of
suspensions in m. 3 that leads to the
secondary dominant (A major) that leads to the
dominant (D major), which then leads back to
tonic (G minor). The 5th measure begins with a
ii-V-I progression and leads to a C
dominant seventh chord. This marks the beginning of a series of
modulations moving up a fourth each time. He modulates twice, arriving in B major for the recapitulation of the opening
theme in m. 1. A G dominant seventh chord is used in the fourth beat of m 8. Measure 9 marks the beginning of a sequence that lasts two measures and further establishes C minor as tonic. There is another recapitulation of the opening measure in mm. 11, this time in C minor. There is a
diminished triad on beat 3 of mm. 12 and a D dominant seventh on the fourth beat as we prepare to modulate back to G minor. There is another C minor chord on the third beat of mm. 13, which turns into a
Neapolitan sixth when the top line lands on an A on the fourth beat. The next measure begins on D, which leads to G minor for the second beat. Tonic (G minor) is then elaborated until mm. 17. There is another Neapolitan chord that leads to a diminished chord on the raised fourth
scale degree, providing a
leading tone to the D dominant seventh chord with a
4-3 suspension in the soprano. The penultimate measure begins with a
pedal tone that last till the end of the prelude. The G minor chord is turned into a G dominant seventh as it modulates to C minor, then a C diminished triad with the pedal tone G in the bass still. Finally, the
dissonance is resolved and the piece ends with a
Picardy third. in a fugal exposition, in which the second theme is slightly modified
Fugue The
subject of the fugue begins in G minor. The second
voice enters on the pickup to the fourth beat of m. 2, and it begins in the dominant (D minor), even though the first note of the theme is a G in this instance. The third voice enters in m. 5 in the tonic and the fourth a measure later in the dominant once again. The first
episode begins in D and modulates to the B major for the recapitulation of the subject. The second episode is in G minor, and it remains in G minor until the end of the piece with a
Picardy third. == References ==