The haunting five-voice fugue in B-flat minor has several characteristics which are notable. First, it is one of the rare fugues with five voices (there are two in Book 1 of the Well-Tempered Clavier, and none in Book 2). Second, the theme features a dramatic and expressive leap of a minor 9th (the big jump upwards between the 2nd and 3rd notes). This is the only fugue theme to have such a dramatic leap. And finally, at the climactic moment of the piece, it features a mind-blowing hyperstretto. Stretto is the overlapping of the theme voices. Bach takes it to the extreme by overlapping all five voices, offset by only one note! See the diagram for an illustration.