The
rule determines the phonetic form of the plural morpheme has traditionally been
called a morphophonemic rule, because its application is determined by both the
morphology and the phonology. When a morpheme has alternative phonetic forms,
these forms are called allomorphs by some linguists. [z], [s], and [əz]
would be allomorphs of the regular plural morpheme, and determined by rule.
To show how such a rule may be applied, assume that
the regular, productive, plural morpheme has the phonological form /z/, with
the meaning ‘plural’. The regular ‘plural rule’ can be stated in a simple way :
1. insert an [ə] before the plural ending when a
regular noun ends in a sibilant /s/, /z/, /ʃ/, /ʒ/, / tʃ/, or /dʒ/
2. Change the voiced /z/ to voiceless [s] when it is
preceded by a voiceless sound.
If neither (1) nor (2) applies, the /z/ will be
realised as [z]; no segments will be added and no features will be changed.
The plural-information rule will derive the phonetic
forms of plurals for all regular nouns (remember, this plural is /z/):
As we have formulated these rules, (1) must be applied
before (2). If we applied the two parts of the rule in reverse order, we would
derive in correct phonetic forms :
The plural-formation rule illustrates once again that
phonological rules can insert entire segments into the phonemic string: an [ə]
is added by the first rule. It also illustrates the importance of ordered rules
in phonology.
An examination of the rule for the formation of past
tense of verbs in English shows some interesting parallels with the plural
formation of nouns.
The productive regular past tense morpheme in English
is /d/, phonemically, but [d] (column B), or [əd] (column C ) phonetically,
again depending on the final phoneme of the verb to which it is attached.
Column D verbs are exceptions.
The past-tense rule in English, like the
plural-formation rule, must include morphological information. Notice that
after a vowel or diphthong the form of the past tense is always [d], even
though no phonological rule would be violated if a [t] were added, as shown by
the words tight, bout, rote. When the word is a verb, and when the final
alveolar represents the past tense morpheme, however, it must be a voiced [d]
and not a voiceless [t].
There is pausible explanation for the insertion of [ə]
in the past tense of regular verbs ending with alveolar stops (and in nouns
ending with sibilants). Because in English we do not contrast long and short
consonants, it is difficult for English speakers to perceive a difference in
consonantal length. If we added a [z] it would be hard for English speakers to
distinguish between squeeze [skwiz] and squeezes [skwizz]. Similarly, if we
added [d] to load, English speakers would find it hard to hear the difference
between the present tense form [loʊd] and the past tense [loʊdd].
