An examination focuses on several instances in which consonant mutation, whereby the initial consonant of a word or constituent undergoes morphophonological changes, occurs in standard literary Welsh. The syntactic and morphological means by which these mutations take place are considered, with emphasis on how an account of these mutations fits into the modular theory of grammar. The possibility that these mutations are the result of subcategorization or agreement is rejected, and it is concluded that the mutations that appear on the NP objects of Ps are the result of syntactic government; this analysis depends on the margin feature to correctly place mutations. Government is not the correct explanation for mutations triggered by genitive markers. An analysis of the these markers as phrasal affixes that require special syntactic branching and which govern mutations is rejected. These markers are instead edge-located inflections that serve as agreement markers and are associated with mutations. (DJD)