This is a chorale prelude from the famous collection of 18 Great Chorales, also known as the Leipzig Chorales. This is a four-part setting and the melody of the chorale (Cantus firmus) is presented in the soprano in longer note values.
The tune is written in the Bar form (AAB). This is a traditional form of the German Lutheran chorales where the part A (Stollen) is repeated with a different text. Section B (Abgesang) is usually longer than A.
The chorale prelude is written in the key of G major. At the beginning Bach uses Vorimitation technique where the first line of the tune is presented in imitation before the general Cantus firmus enters. The beginning features fugal entries of this phrase first in the soprano (tonic), the alto (dominant), and the bass (tonic).
Phrases 1 and 2 of the tune are connected with an interlude where the tune is present in the tenor part. Then both phrases are repeated exactly from the beginning.
Abgesang is prepared by the 3 fugal entrances of phrase 3 of the tune (tenor, bass, and alto). This time the key is D major and the fugal entries are written in the tonic, dominant, and tonic, respectively.
Before phrase 4 (in D major) Bach writes another set of fugal entries using Vorimitation technique (tenor - tonic, bass - dominant, and alto - tonic).
Phrase 5 is foreshadowed by the alto, tenor, and bass fugal entries in the left hand part and pedals. The piece is concluded by the phrase 6 in the alto (dominant) and soprano (tonic) and a long sustained tonic pedal point on the note G in the soprano.
This is a traditional place to touch the subdominant key and and wrap up the entire composition. It is like an extended Plagal cadence involving the chords of the subdominant (C major) and the tonic (G major) which share the same common note (G).
I hope you enjoy this brilliant piece performed on the organ at St. Thomas church in Leipzig:
If you cannot see the video, click on the title link at the beginning of this article.
By the way, do you want to learn my special powerful techniques which help me to master any piece of organ music up to 10 times faster? If so, download my free video Organ Practice Guide.
Or if you want to learn to improvise in the style of Bach? If so, I suggest you check out my free 9 day mini course in Keyboard Prelude Improvisation.