Comic opera from the first half of the eighteenth century borrowed many of the structural and formal features of the dramma per musica. The arias of early comic opera were almost exclusively set in da capo form, which had become ubiquitous near the end of the seventeenth century. Although commentators and librettists frequently lamented the banality of this inherited convention, it persisted until about 1750, when, over the course of approximately a decade, it was replaced by a much more flexible approach to the formal organization of arias. This article investigates that period of experimentation and identifies the individuals who drove the innovations. I argue that two singers in particular, Francesco Baglioni and Serafina Penna, provided the impetus to break away from da capo form. Their desire for arias that displayed their dramatic and musical abilities to the greatest advantage led the librettist Carlo Goldoni to provide them with textual prompts that required new approaches to musical form. By emphasizing the connection between singers and librettists, I draw attention to the collaborative nature of operatic production. This approach also demonstrates the ways in which musical form, usually considered the purview of the composer, is in fact rooted in the features of the libretto and inspired by the inclinations and abilities of singers.