The
function of phonological rules in a grammar is to provide the phonetic
information necessary for the pronunciation of utterances. We may illusrtate
this point in the following way :
That
is, the input to the P-rules is the phonemic representation; the P-rules
applying to or operate on the phonemic strings and produce as output the
phonemic representation.
The
application of rules in this way is called a derivation. We have given a number
of examples of derivations, which show how phonemically oral vowels become
nasalised, how phonemically unaspirated voiceless stops become aspirated, how
contrastive voiced and voiceless alveolar stops in some dialects of English
merge to become flaps, and how German voiced obstruents are devoiced. A
derivation is thus an explicit way of showing both the effects of a
phonological and rule and the function of phonological rules (which we can
abbreviate as P-rules) in a grammar.
All
the example of derivations we have so far considered show the application of
just one phonological rule. It must be the case, however, that more than one
rule may apply to a word. For example, the word tempest is phonemically
/tempest/ (as shown by the pronunciation of tempestuous [the~mphestʃuəs])
but phonetically [the~mpəst]. Three rules apply to it: the aspiration rule, the
vowel nasalisation rule, and the schwa rule.
We can derive the phonetic from from the phonemic
representation as follows :
We are using phonetic symbols instead instead of
matrices in which the future values are changed. These derivations are
equivalent, however, as long as we understand that a phonetic symbol is a cover
term representing a matrix with all distinctive features marked either + or –
(unless, of course, the feature is non-distinctive, such as the nasality value for
phonemic vowels in English).
