By 1779—a few years before Haydn wrote his Symphony No. 76—the 23-year-old Mozart was eager to break free from the restrictions imposed by his employer in Salzburg, the Archbishop Colloredo. His recent tour westward to Mannheim and Paris had proved decisively important; it apparently stirred a desire to experiment with some of the instrumental forms and styles Mozart had been encountering.

One result was the Sinfonia Concertante, a work that bursts with the joy of exploring new instrumental sound combinations and possibilities. It also marks a sort of turning point, in essence summing up much of what Mozart had achieved to date as an artist. Not long afterward—and in part due to his indulging in such purely pleasurable creative endeavors at the expense of his duties as court organist—he was summarily dismissed by his boss (as he sardonically puts it in a letter, “with a kick on my arse”) and left Salzburg for good to live in Vienna.

The genre here, as the name indicates, is a hybrid between the symphony and the concerto—in the 19th century it would be labeled a double concerto for violin and viola. Yet the Sinfonia Concertante wondrously unifies these several descriptions. Like Haydn, Mozart exploits his modest orchestral ensemble to the maximum; there’s no percussion, nor flutes nor his beloved clarinets, but he divides the violas into two for a richer string blend. The proportions of the opening movement (marked with the epic-sounding tempo “Allegro maestoso”) are generous and expansive, further contributing to the work’s symphonic aspect.

For many, this piece represents the grandest of Mozart’s violin concertos, surpassing the five official ones. At the same time, the viola is no second fiddle here. Mozart’s choice of instrument for the second soloist is telling: Although an excellent violinist, he himself loved to play viola in string quartet ensembles, enjoying the perspective of being “in the middle.” One unforgettable characteristic of the Sinfonia Concertante is the remarkable partnership and equality shared by both soloists and the searingly beautiful sound blend they create. Mozart’s original score even inscribes the viola part in D major, thus requiring the violist to tune the strings up a half step. The intention is to give the usually more reserved viola a certain resonance to offset the violin’s usual limelight-hogging sonority.

The Sinfonia Concertante is in part about an extraordinary abundance of ideas and sonorities that—thanks to Mozart’s art—pour out with a seeming effortlessness, like ripened fruit simply there to be plucked. The opening orchestral exposition makes this clear, as one idea is laid out on top of another until, with a half dozen in the air, one loses track. And more are yet to come as the soloists enter in one of the most sublime passages of all Mozart, soaring out from the background on a sustained high E flat. It’s perhaps no surprise that George Balanchine choreographed a famous ballet to this music, for the duo soloists converse not just with the orchestra at large but also with each other (it’s intriguing, as well, to imagine Mozart’s own voice represented by the viola). This is clear in the many echoing passages he unfolds and in his construction of the cadenzas, expressly written out.

Beyond these instrumental dimensions, there’s yet another. This is the world of opera, of lamenting song, with a hint of archaic Baroque sentiment, which comes to the fore in the sensitive and lengthy Andante, one of Mozart’s relatively rare minor-mode slow movements. Here we find an emotional depth that, as Maynard Solomon speculates in his notable biography, may reflect the composer coping with the recent death of his mother. Specifically, the duality of the violin-viola sound contributes to another aspect of the piece’s stunning beauty: Listen as the solo violin takes up its plaintive aria of grief and the response from the viola, now providing a sudden but believable consolation. The two continue to form a complementary pair as Mozart unfolds his song seamlessly, virtually prefiguring what Wagner would later coin as “infinite melody.”

With the Presto rondo finale, an irrepressibly joyful spirit returns. As Alfred Einstein observes, its “gaiety results principally from the fact that in the chain of musical events the unexpected always occurs first, being followed by the expected.” Or, to return to Hesse’s ethereal Immortals from the novel Steppenwolf, the Sinfonia Concertante ends with their characteristic laughter, which is “laughter without an object…simply light and lucidity.”

—Thomas May