In his lifetime, Wolfgang Amadeus Mozart composed upwards of 600 works—symphonies, operas, sonatas, and concertos—that have fixed him in the pantheon of Western Classical music. His known oeuvre, though, just got one bigger as a German library has unearthed a previously unheard piece he wrote in his youth.

While compiling the new edition of Köchel, an extensive catalogue of the Austrian composer’s work that’s been in circulation for 160 years, researchers at the Leipzig Municipal Libraries discovered a handwritten manuscript dating to around 1780. Though not written in Mozart’s hand, the transcription, in dark brown ink on handmade paper, is believed to be a copy of his composition for a string trio, penned in the mid to late-1760s. It is ascribed to “Wofgang Mozart.”

A manuscript containing a composition by Wolfgang Amadeus Mozart at Leipzig Municipal Libraries. Photo: Sebastian Willnow / picture alliance via Getty Images.

The newly discovered work is a 12-minute piece made up of seven miniature movements, written in C major for two violins and bass. It has been given the title “Ganz kleine Nachtmusik” and catalogued in Köchel as KV 648.

According to Ulrich Leisinger, head of research at the International Mozarteum Foundation and editor of the latest iteration of Köchel, Mozart’s father Leopold maintained a list of chamber works his son composed. While all of these pieces have been lost, he said, “it looks as if—thanks to a series of favorable circumstances—a complete string trio has survived in Leipzig.”

He further noted the piece’s significance in Mozart’s earlier body of work, since “until now, the young Mozart has been familiar to us chiefly as a composer of keyboard music and of arias and sinfonias.”

Johann Nepomuk della Croce, The Mozart Family (1780–81), with Leopold on the left, his daughter Maria-Anna and son Wolfgang Amadeus at the keyboard, and a portrait of his late wife Anna Maria at center. Photo: Universal History Archive / Getty Images.

Per the dating of “Ganz kleine Nachtmusi,” it was composed by Mozart while in his teens. Around this time, the child prodigy had already composed his first symphony and concluded a three-year-long concert tour that took him and his family around the noble courts of Munich, Paris, London, and Amsterdam, among others. Köchel lists the new piece as “written before Mozart’s first trip to Italy.”

A manuscript containing a composition by Wolfgang Amadeus Mozart at Leipzig Municipal Libraries. Photo: Sebastian Willnow / picture alliance via Getty Images.

Leisinger also indicated that the manuscript was “evidently” sourced from Mozart’s sister. “It is tempting to think that she preserved the work as a memento of her brother,” he added. “Perhaps he wrote the Trio specially for her and for her name day.”

“Ganz kleine Nachtmusik” will have its German premiere this weekend, when it will be performed for the first time in centuries at the Leipzig Opera.