This paper traces the origins of a specific cadential type ubiquitous in late-eighteenth-century Spanish keyboard music to the tierce coulé, a late-eighteenth-century French agrément that fills in a descending third by step. Following earlier French practice, Spanish organists began to use the written-out form of the tierce coulé in the 1740s at weak cadential points in their keyboard sonatas. By the 1780s, the tierce coulé had developed into a new type of authentic cadence (the AC3) that begins like an imperfect authentic cadence with ^3 in the melody and continues with a post-cadential stepwise descent from that ^3 to a more conclusive ^1. Finally, the AC3 came to serve an important rhetorical function in the late-eighteenth-century Spanish keyboard sonata as a common marker of the end of the primary theme.