J.S. Bach, BWV 1001, Sonata 1 in G Minor, III - Siciliana in B flat, Tutorial (Part 1)
This video begins by demonstrating how, once again, Bach can use the simplest compositional devices, such as an ascending arpeggio and a descending scale, to create a most beautiful little motif.
Taking that one step further, what follows is an unpretentious walking bass. Bach is essentially writing a little dance—but for Bach, simple never means trite, and this piece is extremely touching.
We can see the influence of Bach's younger days as a chorister in the writing of this piece; there is some beautiful dovetailing of the lines, as would happen in choral music.
This video, part-one, only covers the first eight measures, discussing how Bach establishes his key, and then leaves it.