Mozart's autograph manuscript for Nos. 1, 3, and 6 of these duos is extant, providing a date of 27 July 1786. What Mozart does not indicate are the instruments for which these pieces were intended, a subject that has long been debated by scholars. Köchel's catalog lists them as works for French horns, but other authorities (including Alfred Einstein) have opted for basset horns. More recently, the fact that all twelve are in C major has prompted scholars to believe that they must have been composed for brass instruments, horns therefore being the only real possibility. However, experts have been reluctant to accept the horn as the intended instrument, citing the exceptional range covered and the use of chromatic notes, which rarely figure in Mozart's writing for the valveless (natural) horn. Nevertheless, all the notes are available to players skilled in stopping, as was Joseph Leutgeb, Mozart's old friend from Salzburg, the player for whom he composed the four horn concertos. Interestingly, the last of these, K. 495 in E flat, had been completed exactly one month earlier. It therefore appears possible that these pieces, which would have been written for the private pleasure of the players rather than public performance, may have been intended for Leutgeb as a footnote, so to speak, to the concerto.

The twelve duos, all short single-movement works, are as follows: No. 1 Allegro; No. 2 Menuetto; No. 3 Andante; No. 4 Polonaise; No. 5 Larghetto; No. 6 Menuetto; No. 7 Adagio; No. 8 Allegro; No. 9 Menuetto; No. 10 Andante; No. 11 Menuetto; No. 12 Allegro.

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