2 The Role of Distinctive Features in Phonological Rules
For the most part, phonological rules can be expressed with a formula of the form
A -+ B / C D, where the alphabetic symbols represent one or more
phonemes (see appendix 1). However, since the smallest isolatable units of a
language's sound system are not phonemes (or their allophones), but the distinctive
features that compose the phonemes, phonological rules are better stated in terms
of these distinctive features.
To begin to see why this is so, let us return to the Vowel-Lengthening Rule of
English discussed in appendix 1:- English Vowel-Lengthening Rule
[vowel] -+ [lengthened] / [voiced consonants]
The form of rule 1 already anticipates the point that such rules are best expressed
in terms of the distinctive features that make up the phonemes that participate in
the phonological regularities of a language. Contrast rule 1 with the same rule
expressed in terms of phonemes:
Although rules 1 and 2 account for the same data, rule 1 expresses the
generalization that explains why the phonemes that are listed in rule 2 pattern
together. For example, as far as the list of phonemes in rule 2 is concerned, we
could replace z with s on the right (part D of the context), and rule 2 would be
almost the same in form and complexity. Only rule 1 explains why s is excluded in
part D of the rule, however: s is a voiceless consonant, and all of the other
consonants in the right-hand list in rule 2 are voiced. Rule 1 rules out the presence
of s in the list of conditioning phonemes-exactly the right result.
In addition, the formulation of rule 1 makes unnecessary the large number of ai
U^2
individual statements that would be required if we were forced (for some reason) g
to make all of the allophonic statements for each phoneme individually. Clearly, -^2
we would not want to have a rule stating that the phoneme /I/ has a variant [I:] n E
before d, n, z, and so forth. It is not the case that each phoneme of a language - G
must have its own individual sets of rules that determine its allophones; rather, 2 -(^0 0) CV
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