information:highlighting, information:filtering, structural:summarization, linguistic:voice_change
Allomorphy

Above, morphological rules are described as analogies between word forms: dog is to dogs as cat is to cats and dish is to dishes. In this case, the analogy applies both to the form of the words and to their meaning. In each pair, the first word means "one of X", and the second "two or more of X", and the difference is always the plural form -s (or -es) affixed to the second word, which signals the key distinction between singular and plural entities.

One of the largest sources of complexity in morphology is that the one-to-one correspondence between meaning and form scarcely applies to every case in the language. In English, there are word form pairs like ox/oxen, goose/geese, and sheep/sheep whose difference between the singular and the plural is signaled in a way that departs from the regular pattern or is not signaled at all. Even cases regarded as regular, such as -s, are not so simple; the -s in dogs is not pronounced the same way as the -s in cats, and in plurals such as dishes, a vowel is added before the -s. Those cases, in which the same distinction is effected by alternative forms of a "word", constitute 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.