As discussed above, phonemes are abstract sound units stored in the mind, while allophones are the actual pronunciations in speech. What phoneme is realized by what allophones in what specific context is another major question in phonology. The allophonic features (non-distinctive) are predictable. The regularities that what sounds vary in what ways in what context are generalized and stated in phonology as rules. We will discuss some English phonological rules.
Earlier in this chapter, we looked at the allophones of /l/. When the phoneme /l/ appears after a voiceless consonant, it is devoiced. In fact, this phenomenon of devoicing is found in the pronunciation of all the English liquids, glides and the two anterior nasals, as in play, pray, snow, smart, sweet. We can state the devoicing rule as:
(i) Devoice a voiced consonant after a voiceless consonant
Phonologists often use a shorthand notation to write the rules, just like formulas used by scientists. This rule can also be written as:
[+voiced +consonantal] → [–voiced] / [–voiced +consonantal]
Rules stated in words or represented formally express the same regularity. The rules contain three parts: class of sounds affected; phonetic change; phonemic environment.
Nasalization of English vowels is also rule-governed. When we pronounce time, bomb, bean, down, the vowels are all nasalized. But in might, mop, beat, night, the vowels are not nasalized. We can thus generalize:
(ii) Nasalize vowels before nasals.
Both (i) and (ii) are assimilation rules. The features of the sounds affected are changed by the neighboring sounds. In (i) the preceding sound influences the following sound. This kind of assimilation is called progressive assimilation. In (ii), the following sound affects the preceding sound, this kind of assimilation is called regressive assimilation.
In American English, we find the pronunciation of alveolar stops in words like butter, water, writer, rider different from those in words like but, what, write, and ride. Phonologists state a flap rule:
(iii) An alveolar stop becomes a flap when preceded by a stressed vowel and followed by an unstressed vowel.
This English flap rule shows that the voicing contrast between /t/ and /d/ is neutralized in the specific environment.
Aspiration in English is rule-governed. Generally, aspiration occurs when a voiceless stop is the initial of a syllable in which the vowel is stressed. The rule can be stated:
(iv)Voiceless stops are aspirated when they occur initially in a stressed syllable.
Having discussed these rules, we can see that there is no one-to-one
relation between phonemes and allophones. One phoneme may be realized
by many allophones. Different phonemes may be realized by one phone. In
English, all vowels may be changed into a schwa (the sound /
/)
when they are unstressed. For example, the initial vowel of analyze
becomes a shwa in analysis, and the diphthong in phone
also becomes a shwa in phonetic. The phonemic representation
and the phonetic representation of the following sentence can show the
complex relationship.
Betty cried as she left the smoky plane.
Phonemic representation: /beti kraid 
z
i: left 
i:
smuki plein/
Phonetic representation: [beDi khrai
z i:
left
s
uki
phlein]
