In the lexicon of a language, each word is represented in its underlying, or basic, form, which discounts all of the alternations in pronunciation that are predictable by phonological rules. For example, there are phonological rules that will account for the variations in the placement of stress and the alternations of vowel quality that occur in sets of words such as harmOny, harmOnic, harmOnious and melOdy, melOdic, melOdious. The rules that predict the pronunciation of the capitalized O’s are general, rather than specific for each word, and the grammar should state such rules so that the regularities are revealed. Accordingly, each of these words must be entered in the lexicon in a way that represents simply its underlying form, and that allows the alternations that occur to be generated by phonological rules. The underlying form is known as the phonemic—sometimes morphophonemic, or phonological—representation of the word. The phonemes of a language are the segments that contrast in the underlying forms. American English may be said to have at least 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), but others believe that these can be derived from the same underlying vowel as that in the word bud. Phonemes are traditionally written between slanting lines, as /P/, /M/, or /L/.

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. Allophones are conventionally written inside brackets—e.g., [p] or aspirated [p^h].

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 formation; the rules for the possessive forms of nouns and for the 3rd person singular of the present tense of verbs are similar in this respect. The rules are more explanatory, however, if they show that these phonemes behave in a similar way because they form a natural class, or set, whose members are defined by a common property. In the case of these plural forms, the phonemes are all, and only, those that have a high-frequency fricative component; they may be called the sibilant, or strident, phonemes.

Other phonological rules that refer to the natural classes of phonemes have already been mentioned. The rule concerning voiceless stops’ being aspirated in some circumstances and unaspirated in others refers to the subset of phonemes that are both voiceless sounds and stops. Similarly, the variations in vowel length in cat and cad can be expressed with reference to the set of phonemes that are vowels, and also to the set that comprises both voiceless sounds and stops.

Features

Each of the phonemes that appears in the lexicon of a language may be classified in terms of a set of phonetic properties, or features. Phoneticians and linguists have been trying to develop a set of features that is sufficient to classify the phonemes in each of the languages of the world. A set of features of this kind would constitute the phonetic capabilities of man. To be descriptively adequate from a linguistic point of view, the set of features must be able to provide a different representation for each of the words that is phonologically distinct in a language; and if the feature set is to have any explanatory power it must also be able to classify phonemes into appropriate natural classes as required in the phonological rules of each language.

In the earlier work on feature sets, emphasis was placed on the fact that features were the smallest discrete components of language. Not much attention was paid to their role in classifying phonemes into the natural classes required in phonological rules. Instead, they were considered to be the units to which a listener attends when listening to speech. Features were justified by reference to their role in distinguishing phonemes in minimal sets of words such as bill, pill, fill, mill, dill, sill, kill.