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Published online by Cambridge University Press: 01 January 2026
Some phonologically significant generalizations result from processes, often formalized as rewrite rules, while others result from interactions among independently motivated processes, often formalized in terms of serial ordering. We adopt these general formalizations of processes and interactions to address two questions. One is the interaction question: what are all the possible forms of interaction between two processes? The other is the opacity question: what makes an interaction between two processes opaque? We show that these questions are best addressed with a rigorous algebraic formalization of processes and their pairwise interactions, describing the complete formal typology of process interactions and identifying the formal properties of those interactions that lead to different types of opacity.
We thank Matt Gordon and two anonymous reviewers for comments and suggestions that have significantly improved both the form and the substance of this paper. Any and all remaining errors are ours. The first author acknowledges support from NSF BCS Award #2021149.