Phonemes are not physical sounds. They are abstract mental representations of the phonological units of a language, the units used to represent the forms of words in our mental lexicons. These phonemic representations of words, together with the phonological rules of the language, determine their pronunciation.
If phonemes are not the actual sounds, what are they ? We can illustrate the difference between a phoneme and a phonetic segment, called a phone, by referring to the difference between oral and nasalised vowels in English. Both oral and nasalised vowels occur phonetically in English. The following examples show this.
Nasalised vowels occur in English syllables only before nasal consonants. If one substituted oral vowel for the nasal vowels in bean ans roam, the meanings of the two words would not be changed. Try to say these words keeping your velum up until your tongue makes the stop closure of the [n] or your lips come together for the [m]. It will not be easy because in English we automatically lower the velum when producing vowels before nasals in the same syllable. Now try to pronounce bead and robe with a nasal vowel. People would still understand [bi`d] as bead although your pronunciation would probably be interpreted as being very nasal, which it would be. In other words, nasal and oral vowels do not contrast in English. There is just one set of English vowel phonemes despite the fact that there are two sets of vowel phones the set of oral vowels and the set of nasal vowels.
There is a general principle or rule in the phonology of English that tells us when nasalised vowels occur always before nasal consonant, never before oral consonants. The oral vowels in English differ phonemically form each other whereas the differences between the oral vowels and their counterparts do not. There is no principle or rule to predict when, foe example, [i] occurs instead of [e] or [u] or [a] or any of the other vowel phonemes. We must learn that [i] occurs in beam [bi`m] and [bean [bi`n] or that the nasalised [u] occurs in boom [bu`m] or boon [bu`n]. Rather, we generalise from the occurrences of oral and nasalised vowels in English, and form a mental rule that applies to [i] and [u] and all other vowels, that automatically nasalises them before nasal consonants.
The rule, or general princilpe, that predicts when a vowel phoneme will be a nasalised phone is exemplified in table 7.1.
As the words in table 7.1 illustrate, oral vowels in English occur in final position and before non-nasal consonants; nasalised vowels occurs only before nasal consonants. The 'non-words' show us that nasalised vowels do not occur finally or before non-nasal consonants. Therefore oral vowels and their nasalised counterparts never contrast.
Most speakers of English are unaware that the vowels in bead and bean are different sounds. This is because speakers are anware of phonemes, not the physical sounds (phones) which they produce and hear.
Since nasalised vowels do occur phonetically but not phonemically, we can conclude that there is no one-to-one correspondence between phonetic segments and phonemes in a language. One phoneme may be realised phonetically (that is, pronounced) as more than one phone phonetic segment. A phoneme may also be represented by ony one phone.
The different phones that are the realisations of a phoneme are called the allophones of that phoneme. In English, each vowel phoneme has both an oral and a nasalised allophones. The choice of the allophones is not random or haphazard; it is rule governed. No one is explicitly taught these rules. They are learned subconsciously when the native language is acquired. Language acquisition, to a certain extent, is rule construction.
To distinguish between a phoneme and its allophones (the phonetic segments or phones that symbolise the way the phoneme is pronounced in different context), we will use slashes / / to enclose phonemes and continue to use the square brackets [ ] for allophones or phones, for example, [i] and [i`] are allophones of the phoneme /i/; [ɪ] and [i`] are allophones of the phoneme /ɪ/, and so on. Thus we will represent bead and bean phonetically as /bid and /bin/. The rule of the distribution of oral and nasal vowels in English shows that phonetically these words will be pronounced as [bid] and [di`n], respectively. Words are stored in our mental dictionaries in their phonemic form. We refer to these as phonemic transcriptions. The pronunciations of these words are given in phonetic transcriptions, between square brackets.
The function of phonemes is to contrast meanings. Phonemes in themselves have no meaning, but when combined with other phonemes they constitute the forms by which meanings of words and morphemes are expressed.
