From somewhere on the internet:
"JOHANN SEBASTIAN BACH left an astonishing musical legacy when he died in 1750 - including the six Brandenburg Concertos, the B-Minor Mass, the Well-Tempered Clavier, and hundreds of sacred cantatas. Yet no piece has engendered so much controversy as Art of Fugue, Bach's definitive exploration of the art of fugal counterpoint.
Bach and the fugue have become so closely associated that Art of Fugue, as Bach's ultimate statement on the subject, commands great interest in and of itself. And yet there's more. Written in Bach's last years, while he slowly went blind, Art of Fugue trails off unfinished in the climactic four-part fugue which would have crowned the work as well as his career. And woven inside it, musical notation spelling out his name . . . B A C H.
Of such things are legends born, and scholars and performers have been arguing about it ever since. When did Bach actually write Art of Fugue? Did he intend it to be played and listened to for pleasure, or is it an"abstract" work of interest only to music scholars? Did Bach's failing health account for the unfinished movement, or is it Bach's idea of a musical puzzle? The issues are far from settled."
What you heard is something called contrapunctus (Contrapunctus I)
"…Counterpoint is a moderate and reasonable concord which arises when one tone is placed opposite another, from which also the term contrapunctus, that is 'note against note', can be derived. Counterpoint is therefore a combination of tones. If this combination or mixture sounds sweetly to the ears, it is called consonance; if, on the other hand, it sounds harsh and unpleasant, it is called dissonance…"
Compiled by 沉吟至今 aka GoliathGrouper