The middle movement of the A major quartet is also in variations--more earnest and careful on the whole--the precursor of the variations in Haydn's "Kaiser" and Beethoven's A major quartets. These quartet variations far surpass the pianoforte variations in character and workmanship; they consist not merely of a graceful play of passages, but of a characteristic development of new motifs springing from the theme. The success of the quartets, on which Mozart put forth all his best powers, was scarcely sufficient to encourage him to make further attempts in the same direction; not until August, 1786, do we find him again occupied with a quartet (D major, 499 K.), in which may be traced an attempt to {LATER QUARTETS, 1786-1790.} (15) meet the taste of the public without sacrificing the dignity of the quartet style. It is not inferior to the others in any essential point. The technical work is careful and interesting, the design broad--in many respects freer than formerly--the tone cheerful and forcible throughout, with the sentimental element in the background, as compared with the first quartets. The last movement approaches nearest to Haydn's humorous turn of thought, following his manner also in the contrapuntal elaboration of a lightly suggested motif into a running stream of merry humour. Nevertheless, this quartet remained without any immediate successor; it would appear that it met with no very general approval on its first appearance. "A short serenade, consisting of an allegro, romance, minuet and trio, and finale" in G major, composed August 10, 1787 (525 K.), does not belong to quartet music proper. The direction for violoncello, contrabasso, points to a fuller setting, which is confirmed by the whole arrangement, especially in the treatment of the middle parts. It is an easy, precisely worked-out occasional piece. During his stay in Berlin and Potsdam in the spring of 1789 Mozart was repeatedly summoned to the private concerts of Frederick William II. of Prussia, in which the monarch himself took part as a violoncellist. He was a clever and enthusiastic pupil of Graziani and Duport, and he commissioned Mozart to write quartets for him, as he had previously commissioned Haydn[22] and Boccherini,[23] rewarding them with princely liberality. In June of this year Mozart completed the first of three quartets, composed for and dedicated to the King of Prussia, in D major (575 K.); the second, in B flat major (589 K.), and the third, in F major (590 K.), were composed in May and June, 1790. From letters to Puchberg, we know {MOZART'S INSTRUMENTAL MUSIC.} (16)