Phonological processes are unidirectional: a feature spreads from a trigger to a target in a consistent direction, either left to right or right to left. But in some languages, two processes targeting the same feature spread in opposite directions. In ATR vowel harmony, this is described as dominance reversal, where [+ATR] harmony operates in one direction, while [-ATR] harmony overrides it in the other. Structurally parallel cases arise outside of vowel harmony, including nasal cluster dissimilation in Gurindji Stanton 2020. These patterns are more formally problematic than they first appear; because the trigger set of one process overlaps with the target set of the other, the grammar must assign contradictory roles to the same segments for the same feature.

This course uses contradirectional spreading as a case study in how the grammar resolves spreading conflicts. We begin with the theoretical problem: what does it mean for two processes to share an application space, and why does this create problems for our analyses? We then work through different empirical cases, such as Turkana, Komo, Liko and Gurindji, examining how different languages resolve the overlap between contradictory grammatical demands on the same structure. The final sessions turn to the formal tools that phonologists have used to handle these patterns, including underspecification and morphological idiosyncrasies, asking what these mechanisms have in common and what that reveals about the architecture of the phonological grammar.