In this episode we concern ourselves with the inner workings of the fugue. The fugue of the D major set from Book 2 of the Well-Tempered Clavier makes for an excellent study. It is made of a pliable, connectible subject which Bach treats as two small motives. These lend themselves to layering, overlapping, and echoing of all kinds. 

The atomic building block of this fugue subject gives it all at once simplicity, harmonic ambiguity, rhythmic ambiguity, and momentum. 

Prelude and fugue no. 5 in D major BWV 874 - Schornsheim