Mozart composed his last three symphonies (nos. 39, 40, and 41) in rapid succession during just six weeks of 1788. There is no evidence he ever heard any of them performed before his death in 1791, and the date of the first performance of Symphony No. 39 is still a question. The three symphonies differ in key, character, and orchestration, but some historians feel they form a set, partly because number 39 uniquely begins with a slow introduction as if it were an overture to something.
Composers of the 18th and 19th century often chose the key of their composition to match the character of the work they wanted to write, for at this time keys were associated with a mood or topic. Further, the ears of many audience members up into the middle of the 19th century were able to hear the key and how the music moved from one to the next and back to the dominant key. The most astute knew when composers took liberties with the usual modulations and cadences. Beethoven would not have written his “Moonlight Sonata” in any key other than C minor; his E-flat piano concerto is known as “The Emperor,” and his E-flat Third Symphony he originally dedicated to Napoleon.
Mozart’s E-flat symphony also exhibits this noble character from the slow introduction into the last movement. The trumpets and timpani add to this character, for trumpets were only optional and timpani not absolutely necessary in the orchestra of Mozart’s time. Each of the three symphonies of which this is the first in the group have a distinct sound. For number 39 Mozart chose to exclude the oboes from the usual woodwind complement thus allowing the clarinets to take a larger role. He was the first composer to recognize the flexibility of technique and wide spectrum of color of the clarinet and employed it in both orchestral as well as chamber music.