Suppose you were writing a grammar of English and wished to include all the generalities that children acquire about the set of phonemes and their allophones. One way of showing what speakers of the language know about the predictable aspects of speech is to include these generalities as phonological rules in the phonological component of the grammar. These are not the rules that someone teaches you at school or that you must obey because someone insists on it; they are rules that are known unconsciously and that express the phonological regularities of the language.
In
English phonology, such rules determine the conditions under which vowels are
nasalised or voiceless stops are aspirated. They are general rules, applying
not to a single sound but to classes of sounds. They also apply to all the
words in the vocabulary of the language, and they even apply to nonsense words
that are not in the language but could enter the language (such as sint, peeg,
or sprag, which would be /sɪnt/, /pig/, and /spag/ phonemically and [si`nt], [phig],
and [spag] phonetically).
There are also less general rules found in all
languages and there may also be exceptions to these general rules. But what is
of greater interest is that the more we examine the phonologies of the many thousands
of languages of the world, the more we find similar phonological rules that
apply to the same board general classes of sounds, like the one we have
mentioned nasals, voiceless stpos, alveolars, labials, and so on.
For example, many languages of world include the rule
that nasalises vowels before nasal consonants. One need not include a list of
the individual sounds to which the rule applies or the sounds which result from
its application. We can state the rule as :
Nasalise a vowel when it is followed by a nasal
consonant in the sanme syllable.
This rule will apply to all vowel phonemes when they
occur in a context before any segment marked [+ nasal], and will add the
feature [+ nasal] to the feature matrix of the vowels.
Another rule that occurs frequently in the world’s
languages changes the place of articulation of nasal consonants to the place of
articulation of the following consonant. Thus, an /n/ will become an [m] before
before a /p/ or /b/ and will become a velar [ŋ] before a /k/ or /g/. When two
segments agree in their place of articulation they are called homorganic
consonants. This homorganic nasal rules occur in Akan as well as English and
many other languages.
Many languages have rules which refer to [+ voiced]
and [- voiced] sounds. Note that the aspiration rule in English applies to the
class of voiceless stops. As in the vowel nasality rule, we did not list the
individual segments in the rule since it applies to all the voiceless stops /p,
t, k/, as well as to /tʃ/.
That we find such similar rules which apply to the
same classes of sounds across languages is not surprising since such rules
often have phonetic explanations and these classes of sounds are defined by
phonetic features. For this reason, such classes are called natural classes of
speech sounds. A natural class is a group of sounds that share one or more
distinctive features.
Children find it easier to learn a rule (or construct
one) that applies to a natural class of sounds; they do not have to remember
the individual sounds, simply the features that these sounds share.
This fact about phonological rules and natural classes
illustrates why individual phonemis segments are better regarded as
combinations or complexes of features than as indissolube whole segments. If
such segments are not be revealed. It would appear that it should be just as
easy for a child to learn a rule such as
1. Nasalise vowels before /p/, /i/, or /z/
As to learn a rule such as
2. Nasalise vowels before /m/, /n/, or /ŋ/
Rule (1) has no phonetic explanation whereas rule (2)
does. It is easier to raise the velum to produce a nasalised vowel in
anticipation of a following nasal consonant than to prevent the velum from
rising before the consonant closure.
A natural classes is a set of phonemes that can be defineed
by fewer features than any of its members. The classes that include the phoneme
/p, t, k, b, g, m, n, ŋ, tʃ, dʒ/ can be defined specifying one feature, [ -
continuant]. The phoneme /p/ requires three feature specifications, as does any
of the other phonemes in this set.
A class of sounds that can be defined by fewer
features than another class of sounds is clearly more general. Thus, the class
of [ - continuant, - nasal] sounds in some sense more natural than the class
that includes all the segments in that class. Try to do this with feature
notation: you will see why such a class is far from natural.
This does not mean that no language has a rule which
applies to single sound or even to a class of stops excluding /p/. One does
find complex rules in languages including rules that apply to an individual
member of a class, but rules pretaining to natural classes occur more
frequently, and an explanation is provided for this fact by reference to
phonetic properties.
A phonological segment may be a member of a number of
classes; for example, /s/ is a member of the class of [+ obstruent]s,
[consonants]s, [+ alveolar]s, [+ coronal]s, [- stop]s, [+ continuant]s, [+
sibiliant]s, and so on.
