A notebook discovered at the **Bibliothèque nationale de France** (BnF) has been identified as one of the most significant discoveries regarding Wolfgang Amadeus Mozart in several decades. The 44-page manuscript contains composition lessons that Mozart gave in Paris in 1778 to Marie-Louise-Philippine de Bonnières de Guînes, a young aristocrat who was skilled at the harp. The notebook was identified in February 2026 by François-Pierre Goy, a conservator in the Music Department at the BnF. The identification was confirmed by Laurence Decobert, a specialist in Mozart's manuscripts, and Armin Brinzing, director of the Bibliotheca Mozartiana in Salzburg. Between May and July 1778, Mozart gave nearly daily composition lessons to Marie-Louise. The notebook includes exercises and seven short pieces for flute and harp. It preserves the intertwined work of the teacher and student, with Mozart proposing ideas, correcting them, and developing them, while Marie-Louise copied, completed, and experimented with them. Mozart was not particularly complimentary about his student. In a letter to his father, Leopold, on May 14, 1778, he criticized her for having "no ideas." In a later letter, he was equally severe about her compositional abilities. These comments shaped the perception of Marie-Louise as a technically skilled but uninspired figure in Mozart's biography. The manuscript allows for a reassessment of this judgment. Several exercises in the notebook are entirely in Marie-Louise's handwriting. Six of the pieces for flute and harp are completed, based on Mozart's ideas, while the seventh remains unfinished, possibly due to her marriage on July 26, 1778. The notebook does not indicate how far Marie-Louise's talent could have developed, but it clearly shows her commitment to learning composition. For women in Marie-Louise's social circle, musical life largely took place in the private sphere. They learned to play instruments and performed for guests. Some purchased sheet music, commissioned works, or hired professional musicians. These gatherings could be important for musicians' careers, offering opportunities to gain recognition from patrons or secure commissions. Marie-Louise's handwriting is particularly valuable, as it allows tracing her participation in the lessons. Her working method can be followed through the exercises, rather than relying solely on Mozart's judgment of her abilities. For many other women, the remaining traces are much more sparse, such as a single sheet of music or a mention in a letter. This reality aligns with research on European musical culture in the 19th century, particularly on salons—private gatherings where musicians performed for influential patrons and formed relationships with them. As musicologist Rebecca Cypess explains in Women and Musical Salons in the Enlightenment, salons connected the public and private spheres. Hostesses had cultural influence, choosing who to invite and which works to perform, at a time when official musical institutions offered few opportunities for women. This influence relied on privileged positions, such as Marie-Louise's access to Mozart's teaching due to her family's wealth and social status. Nevertheless, the music played in elite homes contributed to the development of musical culture, even though history has often focused on the names of famous composers. The recently discovered compositions were performed publicly for the first time at the BnF on June 21, 2026, by flutist Mathilde Calderini and harpist Nicolas Tulliez. The concert was recorded and broadcast by France Musique.