quality:correction, structural:expansion
Allomorphy

Phonological rules constrain the sounds that can appear next to each other in a language, and morphological rules, when applied blindly, would often violate phonological rules by resulting in sound sequences that are prohibited in the language in question. For example, to form the plural of dish by simply appending an -s to the end of the word would result in the form *, which is not permitted by the phonotactics of English. To "rescue" the word, a vowel sound is inserted between the root and the plural marker, and results. Similar rules apply to the pronunciation of the -s in dogs and cats: it depends on the quality (voiced vs. unvoiced) of the final preceding phoneme.