We often search through composers’ words in vain, unable to find enough information on their motivation and intentions in writing a particular piece. How lucky, then, to have Mozart’s letter from Munich on 6 October 1777 to give us the rationale behind his decision to write sonatas for keyboard and violin: “I enclose for my sister six duets for keyboard and violin by Schuster, which I have often played here. They aren’t bad. If I stay on, I’ll write six myself in same style [gusto], which is very popular here.” Within this short passage we learn that Mozart played chamber music by other composers; that Mannheimers liked the so-called “accompanied sonatas” as well, because Mozart continued to play them there (Mannheim, 8 November, at Wendling’s: “I played three duets with violin accompaniment which I had never seen and the composer of which I had never even heard of”); that Mozart’s impetus in writing the pieces was to compete with, improve upon, and gain by the popularity of pieces by another composer. The genre had many devotees in Mannheim and especially Paris: the first pieces Mozart had published as a child were sonatas for keyboard with violin, in Paris, (Op.1, two sonatas), London (Op.2, two sonatas), and The Hague (Op.3, six sonatas) at ages eight, nine, and ten, respectively; then, content to leave the violin as simple accompaniment to the keyboard, he put the genre away for over a decade.