'''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.Sadie, Stanley. Mozart: The Early Years, 1756-1781, p. 503-505, New York: W.W. Norton & Co. (2006) ISBN 0-393-06112-4.
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:
# Adagio maestoso - Allegro con spirito
# Concertante: Andante grazioso in G major
# Rondeau: Allegro ma non troppo in G major
# Andantino in D minor
# Minuetto - Trio 1 & 2
# 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.