fugue (third theme), although adapted to the organ. There are the conventional dotted rhythms of an ouverture, but the alternation of themes owes more to the tradition of contrasting passages in organ compositions than the solo-tutti exchanges in a Vivaldi concerto. Originally possibly written in the key of D major, a more common key for a concerto or ouverture, Bach might have transposed it and the fugue into E major because Mattheson had described the key in 1731 as a "beautiful and majestic key" avoided by organists. The piece also has three separate themes (A, B, C), sometimes overlapping, which commentators have interpreted as representing the Father, Son and Holy Ghost in the Trinity. Other references to the Trinity include the three flats in the key signature, like the accompanying fugue. As the prelude progresses, the reprises of the first theme are shortened, as in a typical Vivaldi concerto; that of the second theme is simply transposed to the dominant; and those of the third theme become more extended and developed. There are no toccata-like passages and the musical writing is quite different from that of the period. For each theme the pedal part has a different character: a baroque basso continuo in the first theme; a quasi-pizzicato bass in the second; and a stile antico bass in the third, with notes alternating between the feet. All three themes share a three semiquaver figure: in the first theme in bar 1, it is a figure typical of a French ouverture; in the second theme in bar 32, it is an echo in the galant Italian style; and in the third theme in bar 71, it is a motif typical of German organ fugues. The three themes reflect national influences: the first French; the second Italian, with its galant writing; and the third German, with many elements drawn from the tradition of North German organ fugues. The markings of forte and piano in the second theme for the echos show that at least two manuals were needed; Williams has suggested that perhaps even three manuals could have been intended, with the first theme played on the first keyboard, the second and third on the second and the echos on the third.First theme: God, the FatherThe first theme has the dotted rhythms, marked with slurs, of a French ouverture. It is written for five parts with complex suspended harmonies. The first reprise