This is not the case in all languages. In French, nasality is a distinctive feature for both vowels and consonants: gars pronounced [ga], 'lad', contrasts with gant [gã], which means 'glove', and bal [bal], 'dance', contrast with mal [mal], 'evil/pain'. In chapter 6, other examples of French nasalised vowels are presented. Thus, French has both oral and nasal consonant phonemes, but only oral vowel phonemes.
In Ghanaian language Akan (often known as Twi), nasalised and oral vowels occur both phonetically and phonemically; nasalisation is a distinctive feature for vowels in Akan, as the following examples illustrate :
These examples show that vowel nasalisation is not predictable in Akan. As shown by the last minimal pair [pam] / [pãm] there is no rule that nasalises vowels before nasal consonants. Unlike English, oral and nasal vowels contrast before oral consonants, and oral and nasalised vowels (after identical initial consonants) contrast in word-final position. The change of form the substitution of nasalised for oral vowels, or vice versa changes the meaning. Both oral and nasal vowel phonemes must therefore exist in Akan.
Note that two language may have the same phonetic segments (phone) but have two different phonemic systems. Both oral and nasalised vowels exist in English and Akan phonetically; English has no nasalised vowel phonemes, but Akan does. The same phonetic segments function differently in the two languages. Nasalisation of vowels in English is redundant and non-distictive; nasalisation of vowels in Akan is non-redundant and distinctive.
Another non-distinctive feature in English is aspiration. In chapter 6 we pointed out that in English, both aspirated and unaspirated voiceless stops occur. The voiceless aspirated stops [ph, th, kh] and the voiceless unaspirated stops [p,t,k] are in complementary distribution in English, as shown in the following chart :
Where the unaspirated stops occur, the aspirated do not, and vice versa. In addition, although they do not contrast, the one set does not occur where the other set does, as shown by the non-words. One can say spit with an aspirated [ ph], as [sph ɪt], and it would be understood as spit, but your listeners would probably think you were spitting out your words. Given this distribution, we see that sapiration is a redundant, nom-distinctive feature in English; aspiration is predictable, occuring as a feature of voiceless stops in the specified phonemic environments.
This reason speakers of English (if they are not analysing the sounds as linguists or phoneticians ) usually consider the [ ph] in pill and the [p] in spill to be the 'same' sound, just as they consider the [i] and [i~] that represent the phoneme /i/ in bead and bean to be the 'same'. They do so because the difference between them, in this case the feature aspiration, is predictable, redundant, non-distinctive, and non-phonemic (all equivalent terms).
The distribution of aspirated and un aspirated stops is a fact abou English phonology. There are two p sounds (or phones) in English, but only one /p/ phoneme. (This is also true of /t/ and /k/.)
This illustrates why we referred to the phoneme as an abstract unit. We do not utter phonemes; we produce phones, the allophones of the phonemes of the language. /p/ is a phoneme in English that is realised phonetically (pronounced) as either [p] or [ ph]. [p] and [ ph] are allophones of the phoneme /p/.
