Sonata in E-Flat Major, Op. 76/6, Hob. III:80

F.J. Haydn: Sonata in E-Flat Major, Op. 76/6, Hob. III:80 was written in 1797, originally as the String Quartet in E-flat Major, Op. 76, No. 6 (Hob. III:80). Haydn did not personally transcribe this work for the flute. It was published as a "Sonata for Flute and Piano" (often including a violin option) by Artaria (Vienna) and Clementi (London) between 1803 and 1805.
In 1797, Haydn was fulfilling a commission for Count Joseph Erdődy. This resulted in the Op. 76 Quartets, arguably the greatest set of string quartets ever written. Because the Count had "exclusive rights" to the music for two years, the public (and flute players) were desperate to get their hands on these works, leading to a flurry of arrangements as soon as the exclusivity period ended.
1797 was the year Haydn composed the "Gott erhalte Franz den Kaiser" (God Save Emperor Francis), which became the Austrian national anthem. He was at the height of his patriotic fervour and international fame. This E-flat Sonata (Op. 76/6) reflects a composer who felt completely free to experiment with form and harmony, as he had nothing left to prove to the musical world.
In 1797, Haydn was pushing the boundaries of tonality. The second movement of this specific work is famous for its "Fantasia" style, which moves through distant keys (like B major) that were considered radical for the late 18th century. For a flautist of that era, playing in these "remote" keys was a significant technical challenge due to the fingering systems of the time.