The piece is written in score which does not make it clear exactly what medium or instrument(s) are called for. It has been performed and recorded by harpsichordists, pianists, organists and string quartets among others, and parts have also been arranged for orchestra.
Each of the fugues use the same, relatively simple, subject in D minor:
The final fugue, left incompleted at Bach's death, includes the so-called BACH motif (the notes B-A-C-H as in German nomenclature, in which B is B flat and H is B natural) just before it stops abruptly without cadence or resolving to a true end. The most famous "completions" of the final unfinished fugue have been composed by Donald Tovey and Ferruccio Busoni.