Though cadences have long been recognized as an aspect of modality, Gallus Dressler's treatise Praecepta musicae poeticae (1563) offers a new understanding of their relationship to mode and structure. Dressler's comments suggest that the cadences in the exordium and at articulations of the text are "principal" to the mode, shaping the tonal structure of the work. First, it is necessary to determine which cadences indicate which modes. A survey of sixteenth-century theorists uncovered a striking difference between Pietro Aron and his followers and many lesser-known theorists, including Dressier. The latter held that the repercussae of each mode were "principal cadences," contrary to Aron's expansive lists. Dressler's syntactical theory of cadence usage was tested by examining seventeen motets by Dressler and seventy-two motets by various early sixteenth-century composers. In approximately three-fourths of the motets in each group, cadences appeared on only two different pitches (with only infrequent exceptions) in their exordia and at text articulations. These pairs are the principal cadences of Dressler's list, and identify the mode of the motets. Observations and conclusions are offered regarding the ambiguities of individual modes, and the cadence-tone usage of individual composers.