More specifically, nasal vowels occur before nasal consonants that follow and are in the same syllable. For most speakers the [oʊ] vowel in the word Roman [roʊ-mə~n] is not nasalised since it occurs before a syllable break symbolised as-, but the [ə~] is because the vowel and the [n] are in the same syllable.
examples of sounds that do not contrast. The substitution of one for the other does not create a minimal pair.
Oral and nasalised vowels in English are also non-distinct sounds. Unlike the [t] and [ʔ], allophones of /t/, the oral and nasalised allophones of each vowel phoneme never occur in the same phonological context. This was illustrated in table 7.1. They complement each other and are said to be complementary distribution. This is further shown in table 7.2.
When oral vowels occur, nasalised vowels do not occur, and vice versa. It is in this sense that the phones are said to complement each other or to be in complementary distribution.
The concept of complementary distribution is illustrated by Clark Kent and Superman, who represent in different form only one person. When Clark Kent is present, Supermen is not; when Superman is present, Clark Kent is not. Clark Kent and Supermen are therefore in complementary distribution, just as [i] and [ɪ~] are in complementary distribution. Of coure there is a difference between their 'distribution' and the two allophones of the phoneme /i/, since Kent and Superman can occur in the same environment (for example talking to Lois Lane) whereas [i] and [ɪ~] never occur in the same environment or under the same conditions. Clark Kent and Superman are thus more familiar to the allophones of /t/ [t] and [ʔ] which do occur in the same environment. The important point is that the concept of two physical manifestations of a single abstract unit is true of both Clark Kent and Superman and of [i] and [ɪ~].
When sounds are in complementary distribution they do not contrast with each other. The replacement of one sound for the other will not change the meaning of the word although it might not sound like typical pronunciation for English. Given these facts about the patterning of sounds in a language, a phoneme can be defined as a set of phonetically similar sounds that are in complementary distribution with each other and do not contrast. A set may, of course, consist of only one member: some phonemes are represented by only one sound, one allophone. When there is more than one allophone in the set, the phones must be phonetically similar, that is, share most of the same phonetic features. In English, the velar nasal [ŋ] and the glottal glide [h] are in complementary distribution; [ŋ] is not found word-initially and [h] does not occur word-finally. But they share very few phonetic features: [ŋ] is a velar nasal voiced stop; [h] is a glottal voiceless glide (or fricative). Therefore, they are not allophones of the same phoneme: [ŋ] and [h] are allophones of different phonemes.
We mentioned that speaker of a language perceive the different sounds of a single phoneme as being one sound. Two sounds that are not phonetically similar would not be so perceived. Furthermore, it would be difficult for children to classify such sounds together as representing one phoneme. The phonetic similarly criterion reflects the ways in which allophones function together and the kinds of generalisations that children can make in acquiring the phonological contrast of the language.
