Prelude and Fugue in G minor, BWV 861


The Prelude and Fugue in G minor, BWV 861, is No. 16 in Johann Sebastian Bach's Well-Tempered Clavier Book I, keyboard music consisting of 24 preludes and fugues in every major and minor key.
Bach's G minor fugue is "insistent and pathetic". The subject also appears in his funeral cantata Gottes Zeit ist die allerbeste Zeit. The subject of the fugue employs a minor 6th leap in the very first measure, then resolves it with a more conventional stepwise motion. Overall, the piece has a foreboding and admonishing tone.

Harmonic analysis

The prelude and the fugue of BWV 861 are set in G minor, requiring a key signature of two flats.

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 that leads to the dominant, which then leads back to tonic. 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 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.

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, 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