In the English language lexicon, each word is represented in its underlying--basic--form, which discounts all the alternations in pronunciation that are predictable by phonological rules. This underlying form is known as the phonemic (or morphophonemic or phonological) representation of the word. The phonemes are the segments that contrast in the underlying forms. American English may be said to have 13 vowel phonemes, which contrast in the underlying forms of words such as bate, bat, beat, bet, bite, bit, bout, but, boat, dot, bought, balm, and boy. Some authorities consider that there are additional vowel phonemes exemplified in the words bush and beaut(y), while others believe that these can be derived from the same underlying vowel as that in the word bud.
The variants of phonemes that occur in phonetic representations of sentences are known as allophones. They may be considered to be generated as a result of applying the phonological rules to the phonemes in underlying forms. For example, there is a phonological rule of English that says that a voiceless stop such as /p/ is aspirated when it occurs at the beginning of a word (e.g. in pin), but when it occurs after a voiceless alveolar fricative (i.e., after /s/), it is unaspirated (e.g. in spin). Thus the underlying phoneme /p/ has an aspirated and an unaspirated allophone, in addition to other allophones that are generated as a result of other rules that apply in other circumstances.
In stating phonological rules, it is necessary to refer to classes of phonemes. Consider part of the rule for the formation of the plural in English: there is an extra vowel in the suffix if the word ends in the same sound as occurs at the end of horse, maze, fish, rouge, church, or judge. The plural forms of words of this kind are one syllable longer than the singular forms. The phonological rules of English could simply list the phonemes that behave in the same way in the rules for plural f...