This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Linguistics. Please check back later for the full article.
Phonotactics is the study of restrictions on possible sound sequences in a language. While many phonotactic constraints hold generally in any given language, more nuanced phonotactic generalizations can apply to morphemes. For example, phonotactic constraints apply differently to morphemes of different types (such as roots vs. affixes). Different phonotactic constraints can hold sounds that belong to the same morpheme as opposed to sounds that are separated by a morpheme boundary. There are also correlations between phonotactic shapes and following certain morphosyntactic and phonological rules, which may correlate to syntactic category, declension class, or etymological origins.
Approaches to the interaction between phonotactics and morphology broadly aim to answer two questions: What is the status of phonotactic constraints that hold for only some morphemes, and how to account for rules that are sensitive to morpheme boundaries. In some theories of phonology, any reference to the specific identities or subclasses of morphemes would exclude a rule from the domain of phonology proper. These rules are either part of the morphology or are not given the status of a rule at all. Other theories allow the phonological grammar to refer to detailed morphological and lexical information. Depending on the theory, phonotactic differences between morphemes may receive direct explanations or be seen as the residue of historical change and not something that constitutes grammatical knowledge in the speaker's mind.