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 No.

The three sonatas K. 309–311 were published as a set 'Opus IV' in about 1782, by Franz Joseph Heina in Paris.

The second theme's melody is gently decorated with syncopation, accompanied by broken chords in the left hand.

A slow cadenza-like passage containing a rapid ascending chromatic scale leads back to the first theme.

In this passage the beginnings of the main theme of the famous second movement of the Piano Concerto No.