This essay explores the use and eventual abandonment of two pitch aggregates – canto per bemolle (signature of a single flat) and canto per bequadro (void signature) – in Barbara Strozzi’s music. Rooted in the two cantus of Guidonian pedagogy, Strozzi’s practice distinguished tonalities according to their bemolle or bequadro signatures, which signaled distinct pitch collections, cadence points, and text affects that she associated with tonal flatness or sharpness. Between the 1640s and 1660s, Strozzi expanded her notated key signatures while maintaining the distinction between flatness and sharpness, but her tonal style never settled into the norms of functional tonality, such as clear tonic/dominant and major/minor oppositions. We must therefore understand Strozzi’s tonal practice as complete and coherent on its own terms, and not as a transition between Renaissance modes and eighteenth-century keys. In doing so, we perceive her flair for vivid, dramatic, and even bizarre text-expressive effects according to the tonal system of her era.