Haydn plays a part, too, in Mozart’s K. 424 – another Haydn, Joseph’s younger brother, Michael, who had already been in the employ of Mozart’s bête noir, Archbishop Colloredo, prior to Wolfgang’s arrival at the Salzburg court.

In 1808, after the death of both composers, there appeared in a biography of Michael Haydn written by one of his pupils: “He [Michael] was ordered by Colloredo to compose duets for violin and viola. He could not, however, deliver them at the appointed time because he became seriously ill... He was threatened with the cancellation of his salary...”

Mozart, who visited his colleague frequently, “sat down and wrote for his friend with such uninterrupted speed that in a few days the [two remaining] duets were finished and delivered under Michael’s name,” according to the biography.

According to H.C. Robbins Landon, in one of his contributions to The Compleat Mozart, “Michael Haydn had apparently intended to compose a series of six violin and viola duets for the archbishop; the Berlin State Library own copies of four such duets (in C, D, E, and F major)... it thus seems possible that Mozart’s two, in G and B-flat [1783], were carefully designed to complete the set by the choice of two keys not already used by Haydn... Among the devices Mozart uses to camouflage his authorship are the chirping grace notes and trills in the opening movement of K. 424.”

Robbins Landon continues: “The Mozartean chromaticism of the introductory Adagio of K. 424... gives way to the opening theme of the following Allegro, equally Mozartean in its great poise and in the seamless flow of the music. Nor is there anything inferior in what follows, the yearning Andante cantabile of the slow movement, a siciliano in 6/8 time.”

The finale is a theme and variations in the lively style of Joseph Haydn’s own duets for violin and viola, which had probably been written a decade earlier and might have served as models for the duets that Michael did write. Here, the viola part is particularly demanding and Mozart might have had his own virtuosity on that instrument in mind when he wrote it.

- Herbert Glass