This video discusses the opening section of the G-minor Fuga, BWV 1001, and demonstrates how Bach establishes his key of G minor.
It is often tempting to look at Bach’s music and marvel at how complex he can be. But in the opening of the G-minor Fuga, BWV 1001, a potentially complex work in itself, we can see how Bach also uses very simple compositional devices, such as whether the music is going up or down, to underlie the structure of this introductory passage.
The video also highlights some of Bach's very effective use of harmony. There are some enthralling moments in this passage that players often skim over, simply by virtue of having heard them so often.