This fugue is one of the last piece to be filled to complete the WTC II. While transposing the ealier version from F major to Ab major, Bach extended the piece by composing the second half afresh (bb.25-50) .
There is another autograph in Berlin, P 274. Musically it is almost identical with the 'London autograph' version.
The version transmitted in the hand of W. F. Bach, which is probably copied from the 'Berlin autograph'. The only notable differences from P 274 are the appoggiaturas added to the first crotchet of the subject entries.