English has many examples of phoneme neutralization. This refers to the environment in which the contrast between phonemes is neutralized. Many of the examples of phoneme neutralization involve vowels but consonants also undergo neutralization.
The English words "hat," "hot" and "hut" have three vowel phonemes. The first word has a low front vowel, the second a low back vowel and the third a mid central one. However, in word final position, this phonemic contrast is neutralized. In the words "sofa," "drama" and "opera," all three words have either a high or low mid central unrounded vowel. This unstressed vowel is very common in English.
The words "hit" and "heat" have the lax high front unrounded and tense high front unrounded vowels. In word final position, this contrast is also neutralized. In the words "baby" and "happy," only the tense vowel occurs. In a few dialects of English such as many of northern England, only the lax vowel occurs word-finally.
The words "pull" and "pool" also exemplify the contrast between lax and tense. In "pull," the high back rounded vowel is lax while in "pool" it is tense. In word-final position, however, only the tense vowel occurs. This is the case in the words "do" and "blue."
The contrast between lax and tense vowels also occurs in "let" and "late." In word-final position, only the tense vowel occurs as in "play" and "they."
The words "hit," "cup" and "ten" have different vowels. However, before the rhotic approximant, this contrast is neutralized. The vowels of "fir," "fur" and "fern" are the same. The notable exception to this rule is certain dialects of Scotland and Ireland in which the contrast is preserved.
The liquids "l" and "r" contrast in many positions such as in "clue," "crew," "call" and "car." However, this is not the case after the voiceless alveolar plosive. In this case, only the rhotic appears as in "train" and "true."
In the English of many speakers such as those of Canada and the United States, the contrast between the voiced and voiceless alveolar plosives is neutralized between a stressed vowel and an unstressed one. The words "medal" and "metal" and "ladder" and "latter" cannot be distinguished by pronunciation alone. This is another example of phoneme neutralization.
Phoneme neutralization is a common process in the languages of the world. English is one language which provides many examples of this phonological process. Many of the English phonemes which undergo neutralization are vowels and they often undergo neutralization in unstressed word-final position.