Celebrating the pipe organ, the King of Instruments
Bach’s contemporaries regarded Art of Fugue to be a practical work - one that would be both studied and played. C.P.E. Bach’s advertisement announcing the first edition in 1751 mentions that Art of Fugue had been “arranged for use at the harpsichord or organ.” Yet until recent decades it has been assumed that Bach intended Art of Fugue as a theoretical work only. How did this change of view come about?
Part of the reason is that the fugue fell out of favor, beginning even in Bach’s own lifetime. One of Bach’s critics wrote in 1737 that Bach’s compositions would be admired more widely if Bach did not “darken their beauty by an excess of art,” a reference to supposedly over-labored and over-ornamented fugal techniques. Bach was really the last exponent of the fugue and, after his death, the form virtually died with him. Art of Fugue met with so little response that the copper engraving plates were sold as scrap.
Bach’s music - and with it the fugue - languished in obscurity for nearly a century. Even Mozart and Beethoven had to be introduced to the beauties of Bach’s fugues by an aficionado, Baron von Swieten, who played Bach’s fugues at his Sunday salons in Vienna; and both composers began to incorporate fugal elements into their compositions.  It was only in the mid-19th century - thanks to a revival begun by Felix Mendelssohn and Robert Schumann - that Bach’s keyboard works, cantatas, and instrumental music made their way back into concert programs. Art of Fugue, however, was still regarded as too abstruse.
The notation system Bach used in composing Art of Fugue led many to believe that the work was merely theoretical. This “open scoring” notation, whereby each voice is written on a separate staff, seemed to imply an abstract work, as did the fact that Bach left no indication about instrumentation. However, although open scoring was already on its way out in Bach’s day, he did use it in other keyboard compositions. The advantage of open scoring was that the organist/student could clearly see the individual voices. Ability to play from open scores died out in the 19th century and this, combined with the fact that two movements (mirror fugues XII and XIII) are not playable by one person at a keyboard, seemed to further confirm that Art of Fugue was “eye music” only. 
 Philipp Spitta, J.S. Bach (1873-1880).