'''Piano Sonata No. 9 in D major''', K. 311 (284c) was written on the composer's stay in Augsburg and Mannheim in November-December 1777, and is contemporaneous with his Sonata K. 309 (Mozart wrote his Sonata K. 310 in the summer of the following year, in Paris). The three sonatas K. 309-311 were published as a set 'Opus IV' in about 1782, by Franz Joseph Heina in Paris.
The work has three movements:
#''Allegro con spirito''
#''Andante con espressione''
A typical performance takes about 15 to 17 minutes.
The first movement is in sonata form. Its first subject has a quasi orchestral opening, and its second subject in the dominant key (A major) is quieter. The development section is almost entirely based on the last four bars of the exposition.
The second movement has an episodic structure A-B-A-B-A-coda. The second theme's melody is gently decorated with syncopation, accompanied by broken chords in the left hand.
Lastly, the most technically demanding movement of the three is a sonata rondo, with a short central episode in B minor (the main key's relative minor). A slow cadenza-like passage containing a rapid ascending chromatic scale leads back to the first theme.
The piece displays mature thematic development.