Introduced by Jean-Jacques Rousseau in his Letter on French music (1753), "unité de mélodie" has commonly been understood as a technical rule asserting the primacy of melody over all the other musical parameters. It is the key concept of Rousseau's musical thought. Yet studies on eighteenth-century formulations of musical unity have paid only scant attention to Rousseau's discussions of it, explaining its presence in his œœuvre as a symptom of the growing influence of the style galant in France. Drawing on Rousseau's autobiographical and theoretical writings, this essay investigates the genesis of the "unité de mélodie," beginning with its rhetorical roots and the influence of Friedrich-Melchior Grimm's notion of the musical "contresens." But Rousseau's own autobiography is also key to understanding his intellectual approach to music. I argue that the "unité de mélodie" originated well before the years 1752–53, and that it is inextricably linked with Rousseau's preoccupation with writing and musical notation, a preoccupation motivated by his search for musical texts that would allow for direct comprehension, whether through listening or through score reading. Finally, in the concept of "unité de mélodie," we can see how Rousseau made a seminal contribution to late eighteenth-century French theoretical discourses on musical periodicity and on the importance of melody in establishing form.