This article analyzes the Adagio opening movement of Fanny Hensel’s sole string quartet by reading the ways in which it dialogues with several genres: the increasingly independent early nineteenth-century slow introduction; the slow first movement; eighteenth- and early nineteenth-century sonata form; and the fantasia. I build this generic context by considering the historical development of the slow introduction and drawing attention to such overlapping identities in two cello sonata movements by Ludwig van Beethoven and in Hensel’s Sonata o Fantasia in G minor for Piano and Cello (1829).