A long-lost letter by Wolfgang Amadeus Mozart has been found in the archives of a prominent French family. Unlike the new piece of Mozart music that recently turned up, this handwritten note, which has featured in numerous Mozart books over the past 103 years, is heading straight to auction.

Online bidding for the four-part “Autograph Letters, Historical Documents and Manuscripts” sale by International Autographs Auctions Europe SL is already underway, but the hammer starts coming down tomorrow in Spain. This letter by Western history’s greatest composer will hit the block on Thursday. It’s estimated to achieve €100,000 to €150,000 ($105,000–$155,000).

The lot’s description notes that only 25 signed Mozart letters, ”several of them incomplete, and most written to his father or other relatives,” have officially gone on sale over the past 50 years. This specimen features slight staining, tears, and creases, but its ink remains exceptionally bold, allowing for a clear read of its noteworthy contents. The text includes Mozart’s earliest known reference to his famous Haydn string quartets—plus a masterclass on how the enterprising 27-year-old talent peddled his wares.

Mozart penned this letter in April 1783. The “Monsieur!” he addressed was Paris-based musician Jean-Georges Sieber—who became the first person to disseminate Mozart’s music four years prior, when his company published the composer’s “Symphony No. 31 in D Major,” also known as the “Paris Symphony.” This letter, written in German, sees Mozart pitching Sieber three finished piano concertos.

An image of the letter. Photo: International Autograph Auctions Europe SL.

“You probably know about my sonatas for pianoforte with the accompaniment of a violin, which I had published here by Artaria and Company,” Mozart began, mentioning his unofficial publishing house, before talking a little trash. “I am not very satisfied with the local engraving,” Mozart intimated, “and even if I were, I would also like to send something to my compatriot in Paris again.”

“So I hereby inform you that I have three piano concertos that can be performed with a full orchestra, with oboes and horns, or just à quatro,” Mozart wrote. Emboldened by the fact that he knew Artaria would take them if Sieber declined, the composer added, “you give me 30 Louis d’or for them, and that is the end of our business.”

Mozart even offered Sieber six quartets, which would eventually become his highly famous Haydn quartets. Only one was done by this point, but Mozart famously wrote music in his head first, so they were likely all, in a sense, nearing completion. “I cannot let these go as cheaply,” Mozart negotiated. “I cannot let you have these 6 quartets for under 50 Louis d’or.”

He concluded by providing an address for Sieber’s response, which is now lost. But, historians have deduced that Sieber turned Mozart down, since Ataria announced the publication of all six quartets in September 1785, after compensating Mozart 100 ducats (about $30,000). Mozart named these works for Franz Joseph Haydn, the father of the string quartet. When Haydn had heard them six months prior, he told Mozart’s father “your son is the greatest composer known to me either in person or by name.”

As of press time, two bids have been placed on the historic letter. The highest is €100,000.