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:

1. Adagio maestoso - Allegro con spirito
2. Minuetto
3. Concertante: Andante grazioso in G major
4. Rondeau: Allegro ma non troppo in G major
5. Andantino in D minor
6. Minuetto - Trio 1 & 2
7. Finale: Presto

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.

Notes

1. ^ Sadie, Stanley. Mozart: The Early Years, 1756–1781, p. 503-505, New York: W.W. Norton & Co. (2006) ISBN 0-393-06112-4.