With the great progress made in phonetics in the late 19th century, it had become clear that the question whether two speech sounds were the same or not was more complex than might appear at first sight. Two utterances of what was taken to be the same word might differ quite perceptibly from one occasion of utterance to the next. Some of this variation could be attributed to a difference of dialect or accent and is of no concern here. But even two utterances of the same word by the same speaker might vary from one occasion to the next. Variation of this kind, though it is generally less obvious and would normally pass unnoticed, is often clear enough to the trained phonetician and is measurable instrumentally. It is known that the "same" word is being uttered, even if the physical signal produced is variable, in part, because the different pronunciations of the same word will cluster around some acoustically identifiable norm. But this is not the whole answer, because it is actually impossible to determine norms of pronunciation in purely acoustic terms. Once it has been decided what counts as "sameness" of sound from the linguistic point of view, the permissible range of variation for particular sounds in particular contexts can be measured, and, within certain limits, the acoustic cues for the identification of utterances as "the same" can be determined.
What is at issue is the difference between phonetic and phonological (or phonemic) identity, and for these purposes it will be sufficient to define phonetic identity in terms solely of acoustic "sameness." Absolute phonetic identity is a theoretical ideal never fully realized. From a purely phonetic point of view, sounds are more or less similar, rather than absolutely the same or absolutely different. Speech sounds considered as units of phonetic analysis in this article are called phones, and, following the normal convention, are represented by enclosing the appropriate alphabetic symbol in square brackets. Thus, [p] will refer to a p sound (i.e., what is described more technically as a voiceless, bilabial stop); and [pit] will refer to a complex of three phones-a p sound, followed by an i sound, followed by a t sound. A phonetic transcription may be relatively broad (omitting much of the acoustic detail) or relatively narrow (putting in rather more of the detail), according to the purpose for which it is intended. A very broad transcription will be used in this article except when finer phonetic differences must be shown.
Phonological, or phonemic, identity was referred to above as "sameness of sound from the linguistic point of view." Considered as phonological units-i.e., from the point of view of their function in the language-sounds are described as phonemes and are distinguished from phones by enclosing their appropriate symbol (normally, but not necessarily, an alphabetic one) between two slash marks. Thus, /p/ refers to a phoneme that may be realized on different occasions of utterance or in different contexts by a variety of more or less different phones. Phonological identity, unlike phonetic similarity, is absolute: two phonemes are either the same or different, they cannot be more or less similar. For example, the English words "bit" and "pit" differ phonemically in that the first has the phoneme /b/ and the second has the phoneme /p/ in initial position. As the words are normally pronounced, the phonetic realization of /b/ will differ from the phonetic realization of /p/ in a number of different ways: it will be at least partially voiced (i.e., there will be some vibration of the vocal cords), it will be without aspiration (i.e., there will be no accompanying slight puff of air, as there will be in the case of the phone realizing /p/), and it will be pronounced with less muscular tension. It is possible to vary any one or all of these contributory differences, making the phones in question more or less similar, and it is possible to reduce the phonetic differences to the point that the hearer cannot be certain which word, "bit" or "pit," has been uttered. But it must be either one or the other; there is no word with an initial sound formed in the same manner as /p/ or /b/ that is halfway between the two. This is what is meant by saying that phonemes are absolutely distinct from one another-they are discrete rather than continuously variable.
How it is known whether two phones realize the same phoneme or not is dealt with differently by different schools of linguists. The "orthodox" post-Bloomfieldian school regards the first criterion to be phonetic similarity. Two phones are not said to realize the same phoneme unless they are sufficiently similar. What is meant by "sufficiently similar" is rather vague, but it must be granted that for every phoneme there is a permissible range of variation in the phones that realize it. As far as occurrence in the same context goes, there are no serious problems. More critical is the question of whether two phones occurring in different contexts can be said to realize the same phoneme or not. To take a standard example from English: the phone that occurs at the beginning of the word "pit" differs from the phone that occurs after the initial /s/ of "spit." The "p sound" occurring after the /s/ is unaspirated (i.e., it is pronounced without any accompanying slight puff of air). The aspirated and unaspirated "p sounds" may be symbolized rather more narrowly as [ph] and [p] respectively. The question then is whether [ph] and [p] realize the same phoneme /p/ or whether each realizes a different phoneme. They satisfy the criterion of phonetic similarity, but this, though a necessary condition of phonemic identity, is not a sufficient one.
The next question is whether there is any pair of words in which the two phones are in minimal contrast (or opposition); that is, whether there is any context in English in which the occurrence of the one rather than the other has the effect of distinguishing two or more words (in the way that [ph] versus [b] distinguishes the so-called minimal pairs "pit" and "bit," "pan" and "ban," and so on). If there is, it can be said that, despite their phonetic similarity, the two phones realize (or "belong to") different phonemes-that the difference between them is phonemic. If there is no context in which the two phones are in contrast (or opposition) in this sense, it can be said that they are variants of the same phoneme-that the difference between them is nonphonemic. Thus, the difference between [ph] and [p] in English is nonphonemic; the two sounds realize, or belong to, the same phoneme, namely /p/. In several other languages-e.g., Hindi-the contrast between such sounds as [ph] and [p] is phonemic, however. The question is rather more complicated than it has been represented here. In particular, it should be noted that [p] is phonetically similar to [b] as well as to [ph] and that, although [ph] and [b] are in contrast, [p] and [b] are not. It would thus be possible to regard [p] and [b] as variants of the same phoneme. Most linguists, however, have taken the alternative view, assigning [p] to the same phoneme as [ph]. Here it will suffice to note that the criteria of phonetic similarity and lack of contrast do not always uniquely determine the assignment of phones to phonemes. Various supplementary criteria may then be invoked.
Phones that can occur and do not contrast in the same context are said to be in free variation in that context, and, as has been shown, there is a permissible range of variation for the phonetic realization of all phonemes. More important than free variation in the same context, however, is systematically determined variation according to the context in which a given phoneme occurs. To return to the example used above: [p] and [ph], though they do not contrast, are not in free variation either. Each of them has its own characteristic positions of occurrence, and neither occurs, in normal English pronunciation, in any context characteristic for the other (e.g., only [ph] occurs at the beginning of a word, and only [p] occurs after s). This is expressed by saying that they are in complementary distribution. (The distribution of an element is the whole range of contexts in which it can occur.) Granted that [p] and [ph] are variants of the same phoneme /p/, it can be said that they are contextually, or positionally, determined variants of it. To use the technical term, they are allophones of /p/. The allophones of a phoneme, then, are its contextually determined variants and they are in complementary distribution.
The post-Bloomfieldians made the assignment of phones to phonemes subject to what is now generally referred to as the principle of bi-uniqueness. The phonemic specification of a word or utterance was held to determine uniquely its phonetic realization (except for free variation), and, conversely, the phonetic description of a word or utterance was held to determine uniquely its phonemic analysis. Thus, if two words or utterances are pronounced alike, then they must receive the same phonemic description; conversely, two words or utterances that have been given the same phonemic analysis must be pronounced alike. The principle of bi-uniqueness was also held to imply that, if a given phone was assigned to a particular phoneme in one position of occurrence, then it must be assigned to the same phoneme in all its other positions of occurrence; it could not be the allophone of one phoneme in one context and of another phoneme in other contexts.
A second important principle of the post-Bloomfieldian approach was its insistence that phonemic analysis should be carried out prior to and independently of grammatical analysis. Neither this principle nor that of bi-uniqueness was at all widely accepted outside the post-Bloomfieldian school, and they have been abandoned by the generative phonologists (see below).
Phonemes of the kind referred to so far are segmental; they are realized by consonantal or vocalic (vowel) segments of words, and they can be said to occur in a certain order relative to one another. For example, in the phonemic representation of the word "bit," the phoneme /b/ precedes /i/, which precedes /t/. But nonsegmental, or suprasegmental, aspects of the phonemic realization of words and utterances may also be functional in a language. In English, for example, the noun "import" differs from the verb "import" in that the former is accented on the first and the latter on the second syllable. This is called a stress accent: the accented syllable is pronounced with greater force or intensity. Many other languages distinguish words suprasegmentally by tone. For example, in Mandarin Chinese the words haò "day" and haǒ "good" are distinguished from one another in that the first has a falling tone and the second a falling-rising tone; these are realized, respectively, as (1) a fall in the pitch of the syllable from high to low and (2) a change in the pitch of the syllable from medium to low and back to medium. Stress and tone are suprasegmental in the sense that they are "superimposed" upon the sequence of segmental phonemes. The term tone is conventionally restricted by linguists to phonologically relevant variations of pitch at the level of words. Intonation, which is found in all languages, is the variation in the pitch contour or pitch pattern of whole utterances, of the kind that distinguishes (either of itself or in combination with some other difference) statements from questions or indicates the mood or attitude of the speaker (as hesitant, surprised, angry, and so forth). Stress, tone, and intonation do not exhaust the phonologically relevant suprasegmental features found in various languages, but they are among the most important.
A complete phonological description of a language includes all the segmental phonemes and specifies which allophones occur in which contexts. It also indicates which sequences of phonemes are possible in the language and which are not: it will indicate, for example, that the sequences /bl/ and /br/ are possible at the beginning of English words but not /bn/ or /bm/. A phonological description also identifies and states the distribution of the suprasegmental features. Just how this is to be done, however, has been rather more controversial in the post-Bloomfieldian tradition. Differences between the post-Bloomfieldian approach to phonology and approaches characteristic of other schools of structural linguistics will be treated below.
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