have noted, at a time when Bach was busy composing counterpoint for the second book of The Well-Tempered Clavier and the Goldberg Variations (Clavier-Übung IV) using a very wide harpsichord range, Bach wrote the duets to lie comfortably in the range C to c″″′ in Helmholtz pitch notation (C2 to C6 in scientific pitch notation), so within the relatively narrow compass of almost every organ of the time. The pieces can nevertheless be played on any single keyboard, such as a harpsichord or fortepiano. The use of the term duetto itself is closest to that given in the first volume of the Critica Musica (1722) of Johann Mattheson: a piece for two voices involving more than just "imitation at the unison and the octave". It was Mattheson's view that "a composer's true masterpiece" could rather be found in "an artful, fugued duet, more than a many-voiced alla breve or counterpoint". In choosing the form of the compositions, which go considerably beyond his Two part inventions BWV 772–786, Bach might have been making a musical contribution to the contemporary debates on the theory of counterpoint, already propounded in the tracts of Friedrich Wilhelm Marpurg and of Johann Fux, whose Gradus ad Parnassum had been translated by Bach's friend Mizler. has suggested that it may have been a direct response to the ongoing argument on musical style between Birnbaum and Scheibe: Bach combines the simple and harmonious styles advocated by his critics Mattheson and Scheibe with a more modern chromatic and often dissonant style, which they regarded as "unnatural" and "artificial". Despite many proposed explanations—for example as accompaniments to communion, with the two parts possibly signifying the two sacramental elements of bread and wine—it has never been determined whether Bach attached any religious significance to the four duets; instead it has been considered more likely that Bach sought to illustrate the possibilities of two-part counterpoint as fully as possible, both as a historical account and "for the greater glory of God". Duetto I BWV 802 The first duet in E minor is a double fugue, 73 bars long, in which all the musical material is invertible, i.e. can be exchanged between the two parts. The first subject is six bars long broken up into one bar segments. It is made up of one bar of demisemiquaver scales leading into four bars where the theme becomes angular, chromatic and syncopated. In the sixth