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 (God's time is the very best time). 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.
The prelude and the fugue of BWV 861 are set in G minor, requiring a key signature of two flats.
Prelude
The first 2 measures of the prelude are an elaboration of tonic. There is a chain of suspensions in measure three 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 2-5-1 progression, but it leads to a C dominant 7th chord. This marks the beginning of a series of modulations moving up a fourth each time. He modulates twice, arriving in B flat major for the recap of the opening theme in mm. 1. A G dominant 7th chord is used in the 4th beat of mm 8. Mm 9 marks the beginning of a sequence that lasts two measures and further establishes C minor as tonic. There is another recap of the opening measure in mm. 11, this time in C minor. There is a diminished triad on beat 3 in mm. 12, and a D dominant 7th on the 4th beat as we prepare to modulate back to G minor. There is another C minor chord on the 3rd beat of mm. 13, it turns into a Neapolitan 6th when the top line lands on an A flat on the 4th 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 7th 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 7th as it modulates to C minor, then a C diminished triad with the pedal tone G in the base 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 4th beat of the second measure, 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 mm. 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 relative major (F 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
