There are many apocryphal stories in the classical-music world, but the one in which Frederick the Great challenged Bach to improvise a six-part fugue on a theme of the king's own invention is true, and The Musical Offering was, after a period of further reflection, the result. As with all the works of Bach's later years, the work is both great art and a "teaching piece," which shows everything that he thought could be done with the king's theme.
The Art of Fugue (theme)
The Trio Sonata based on the theme is the only major piece of chamber music from Bach's last decades in Leipzig, and that makes the work and essential cornerstone of any Bach collection. This performance, led by Neville Marriner, is both polished and lively, and very well recorded. At a "twofer" price, coupled with The Art of Fugue, it's the preferred version of the work on modern instruments. --David Hurwitz