A small notebook recently discovered in the Bibliothèque Nationale de France has been identified as one of the most important Mozart-related finds in decades. The 44-page document contains composition lessons that Wolfgang Amadeus Mozart gave in Paris in 1778 to Marie-Louise-Philippine de Bonnières de Guînes, a young aristocratic harpist. The manuscript was recognized in February 2026 by François-Pierre Goy, a curator at the library, who identified Mozart's handwriting. Laurence Decobert, a Mozart manuscript specialist, and Armin Brinzing, director of the Bibliotheca Mozartiana in Salzburg, later confirmed its authenticity. The notebook provides insight into Mozart's teaching methods and preserves the musical work of both the teacher and his student. Marie-Louise was the daughter of Adrien-Louis de Bonnières, a diplomat and amateur flautist who commissioned Mozart’s Concerto for Flute and Harp, K299, for father and daughter to perform together. Between May and July 1778, Mozart taught Marie-Louise composition almost every day. The notebook contains exercises and seven short pieces for flute and harp. It shows the collaborative nature of the lessons, with Mozart proposing, correcting, and developing ideas, while Marie-Louise copied, completed, and experimented with them. Mozart was not complimentary about his pupil in letters to his father, Leopold, describing her as lacking in ideas. These remarks have influenced historical perceptions of Marie-Louise as a technically capable but uninspired figure. However, the manuscript provides a more nuanced view of her engagement with composition. Several exercises in the notebook are entirely in Marie-Louise's handwriting. Six of the flute-and-harp pieces are complete, beginning with ideas from Mozart, while the seventh remains unfinished, possibly due to her marriage on July 26, 1778, which may have ended her lessons. Marie-Louise's handwriting is valuable as it records her active participation in the lessons. This is significant as the surviving evidence of women's musical contributions is often limited to a single sheet of music or a mention in a letter. The pattern of musical activity in salons, where influential patrons hosted musicians, is similar to research into 19th-century European musical culture. Musicologist Rebecca Cypess argues that salons mediated between public and private spheres, giving hostesses cultural influence at a time when official institutions offered few opportunities for women. Marie-Louise's access to Mozart's works was due to her family's wealth and status. The music played in elite homes contributed to general musical culture, even if later historical studies focused primarily on Mozart. The newly discovered compositions received their first public performance at the Bibliothèque Nationale de France on June 21, 2026, played by flautist Mathilde Calderini and harpist Nicolas Tulliez. France Musique recorded and broadcast the performance.