The vast majority of Wolfgang Amadeus 
Mozart’s symphonic output lacks clarinets. 
This is not because Mozart didn’t like the 
instrument—he quite enjoyed the clarinet—
orchestras at Mozart’s disposal often didn’t have clarinets. The clarinet had not yet become standard in orchestras; it is likely that Mozart first heard the clarinet in London, 1764. Johann Christian Bach introduced the instrument to British audiences the decade prior. After becoming popular in London in the ’50s, the clarinet became widespread in Paris in the 1760s but would not make its way to German or Austrian concert halls until the 1780s. 

Mozart traveled to Paris in 1778 in search of work, and during this time he was exposed to clarinets once more. The Paris Symphony, No. 31, (1778) employs a fully-fortified woodwind section, finally including a pair of the still-rare, single-reed instruments. In a letter home to his father that year, Mozart lamented, “If only we had clarinets!” The young composer would get his wish within the decade. 

Symphony Nos. 32 (1779), 33 (1779), and 34 (1780) have no clarinet parts due to lack of available instruments and dedicated players. Good things come to those who wait, however. In the early 1780s Mozart befriended Anton Stadler, an Austrian clarinetist who was working not only to garner a virtuoso reputation, but to evolve the instrument. Clarinets had finally made their way to Austria. Symphony No. 35, Haffner (1782) proudly boasts two clarinet parts; Mozart and Stadler had worked closely on orchestral clarinet writing in this time. In 1785, Mozart wrote to Stadler: 

“I have never heard the like of what you contrived with your instrument. Never should I have thought that a clarinet could be capable of imitating the human voice as it was imitated by you. Indeed, your instrument has so soft and lovely a tone that no one can resist it . . . .” 

The Prague and Linz Symphonies forgo the instrument once more, but in Symphony No. 39 (1788), Mozart regains his beloved clarinets. He uses them as a mellow alto voice within the woodwind section; clarinet provides a timbral richness that fully links the lower ranged instruments to those at the top of the score. The addition of clarinet into the complement created a truly cohesive woodwind sound and became Mozart’s new orchestral standard. 

In the first movement of Symphony No. 39, woodwind arpeggios showcase the clarinet’s range and agility—impressive for having just five keys—while the harmonies between clarinets illustrate how reliably in tune and blended their sound could be. The principal clarinet begins to serve in a more independent role by shading principal flute lines in the second movement, Andante con moto. The woodwinds begin to take hold of melody as the third movement Minuet repeats, but the clarinets truly take center stage at the onset of Mozart’s Trio. An arpeggiated bass line in the second clarinet’s chalumeau register supports the principal clarinet’s melody two octaves above. In the final movement, principal clarinet grows to its most independent and is now presenting thematic material alongside chirpy interjections by equally soloistic flute and bassoon. 

Mozart continued to highlight the clarinet after Symphony No. 39. The next year, he composed the Clarinet Quintet (1789) for Anton Stadler, which demonstrates the instrument’s lyricism alongside the string quartet. In his opera Clemenza di Tito (1791), Mozart composed a prominent clarinet obligato that parallels the voice in the aria “Parto, parto ma tu ben mio.” Mozart composed his Clarinet Concerto, again for Stadler, in the final year of his life. The monumental work in clarinet repertoire was premiered just months before Mozart’s death in December 1791.