The Serenade for Orchestra No. 9 in D major K. 320, Posthorn, was written by Wolfgang Amadeus Mozart in Salzburg, in 1779. The manuscript is dated 3 August 1779 and was intended for the university "finalmusik" ceremony that year.
The serenade is scored for 2 flutes, flautino, 2 oboes, 2 bassoons, 2 horns, 2 trumpets, post horn, timpani and strings. It has seven movements:
The Concertante and Rondeau movements feature prominent concertante sections for flute and oboe.
The first trio of the second minuet features a solo flautino (or piccolo) played over strings. The second trio of the second minuet features a solo for the post horn. This solo gives the serenade its nickname.
A typical performance lasts approximately 45 minutes.