Die Kunst der Fuge ,
BWV 1080.
The "last" (dating is always tricky with Bach), and,
unfinished work of
J.S. Bach.
A set of 18 fugues (here named by Bach contrapuncti), interspersed with four canons. The work itself is a sort of enigma. For a long time, it was considered only as a didactic device for the study of counterpoint and having no aesthetic value in itself, i.e. not intended for performance. It, in fact, looks like one: there is no instrumentation specified and the fugues are ordered from simple to difficult one.
As it's mostly written on two staves, usually it's played on organ, harpsichord (cembalo), piano etc.
The revival came in 1920s, when Wolfgang Gresser published an orchestral version of his, which roused great interest and disapproval, ultimately driving Gresser to suicide. Since then, many other arrangements were made, and nowadays the set is considered as a masterpiece and, played on pianos, small ensembles, string quartets, etc.
Like Musikalisches Opfer, one theme, variously modified, is used as a basis (of course, counterthemes are used), so the work could be monotonous, more so because it's basically one form - fugue. Yet it is utterly fascinating, never boring and with great emotional depth. I heard a live performance of selections (the thing is very long, takes about an hour and half to perform complete) of Kunst der Fuge interspersed with several miniatures by György Kurtág played by the wonderful Keller Quartet in summer 2000 at late night concert at BBC PROMS in Royal Albert Hall in London and was deeply moved.
Oh yes, the thing is not completed - the last fugue (contrapunctus 18) is actually a fragment of a quadruple fugue with the last part missing. Bach didn't live to finish the piece (he was also almost blind), some people like to find transcendent overtones here, mainly with the use of the B-A-C-H theme.
In the first edition, the chorale „Vor deinen Thron tret ich hiermit" (BWV 668) was used as the ending, added probably by C. P. E. Bach. Nowadays it's usually omitted as it's not really part of the work.
My favourite recording of the work remains the Hesperion XX's effort, directed by Jordi Savall, available on Astrée Auvidis JADE E 2001. The play on period instruments, a consort of viols, with some winds here and there.