Bach’s last major work, this piece was not even entirely completed. It is a difficult piece that takes the form of the fugue to its limits, and requires either patience or familiarity of the listener. Strictly, it is not so much a piece as a series of demonstrations of the fugal form. Each is based around the same theme, but each develops differently, illustrating a different way in which a fugue might be composed.
There is some controversy as to how “The art of fugue” should even be performed. While a keyboard seems the obvious choice, there are fugues in the work that are not playable with just two hands. There seems to be quite a strong case for Bach having intended the Art to be “pure music”, not necessarily intending it for particular performance.