Descartes’ Compendium musicae and Lamy’s La Rhétorique ou l’art de parler, both published in English translation in London in the late seventeenth century, suggest approaches to period performance practice that support expressive intonation as a rhetorical device. Descartes’ unique perspective on musical pitch and intervals provides a methodology for understanding inflected intonation in performance. Closely aligned with Descartes’ epistemological perspective, Lamy’s treatise provides an understanding of expressive intention as essential to effective rhetorical delivery. These approaches are applied to musical examples from trio sonatas of Arcangelo Corelli, John Ravenscroft and Henry Purcell, demonstrating that expressive intonation using subtle pitch inflection can be explained as a rhetorical practice. These subtle pitch inflections, related as they are to both rhetorical delivery and intonation systems, are not reflected in notation but realized only as music is heard in time. It is in performance contexts that pitch inflection can be realized as an expressive device. A supplemental audio file contains five short examples demonstrating pitch deviation applied to selected intervals.