Sor Juana Inés de la Cruz’s Romance 21 “Después de estimar mi amor” sheds light on the author’s musical knowledge with elaborate metaphors that showcase the nun’s theoretical understanding. At the same time, the lyric narrator ironically begs pardon for failing to send her interlocutor a music treatise titled Caracol. While most prior scholarship on Romance 21 focuses on Sor Juana’s engagement with early modern music theory and philosophy, this article addresses the significant but overlooked theme of musical patronage. First, a gendered approach highlights the Caracol as a unique example of a female-authored music theory treatise written for a woman benefactor. Next, an epistolary reading highlights inconsistencies among personal references in the poem and the recipient that its epigraph identifies, the Countess of Paredes. Indeed, details suggest that the addressee is not the Countess of Paredes María Luisa de Lara y Gonzaga, but rather the Countess of Villaumbrosa María Petronila Niño de Porres y Enríquez de Guzmán. For all this, Romance 21 lends notable insight into early modern women’s musical patronage and also advances Sor Juana studies as the first extant communication between the poet and the Countess of Villaumbrosa.