For example, in many languages, including English, most front vowels are unrounded, while most back vowels are rounded. There are no languages in which all front vowels are rounded and all back vowels are unrounded. The most likely explanation for this is that front vowels have a higher second formant (F2) than back vowels, and unrounded vowels have a higher F2 than rounded vowels. Thus unrounded front vowels and rounded back vowels have maximally different F2s, enhancing their phonemic differentiation.
Phonemic differentiation can have an effect on diachronic sound change. In chain shifts, phonemic differentiation is maintained, while in phonemic mergers it is lost. Phonemic splits involve the creation of two phonemes out of one, which then tend to diverge because of phonemic differentiation.
In a chain shift, one phoneme moves in acoustic space, causing other phonemes to move as well to maintain optimal phonemic differentiation. An example from American English is the Northern cities vowel shift, where the raising of /æ/ has triggered a fronting of /ɑ/, which in turn has triggered a lowering of /ɔ/, and so forth.
If a phoneme moves in acoustic space, but its neighbors do not move in a chain shift, a phonemic merger may occur. In this case, a single phoneme results where an earlier stage of the language had two phonemes. A well known example of a phonemic merger in American English is the cot-caught merger, by which the vowel phonemes /ɑ/ and /ɔ/ (illustrated by the words cot and caught respectively) have merged into a single phoneme in some accents.
In a phonemic split a phoneme at an earlier stage of the language is divided into two phonemes over time. Usually this happens when a phoneme has two allophones appearing in different environments, but sound change eliminates the distinction between the two environments. For example in umlaut in the Germanic languages, the back vowels /u, o/ originally had front rounded allophones [y, ø] before the vowel /i/ in a following syllable. When sound change caused the syllables containing /i/ to be lost, a phonemic split resulted, making /y, ø/ distinct phonemes.
It is sometimes difficult to determine whether a split or a merger has happened in cases where one dialect has two phonemes corresponding to a single phoneme in another dialect; diachronic research is usually required to determine which dialect is the conservative and which is the innovative. While some splits and mergers are considered to be part of standard languages, others are not considered standard and may be stigmatized. In descriptive linguistics, however, the question of which splits and mergers are prestigious and which are stigmatized is irrelevant.
Occasionally, speakers of one accent may believe the speakers of another accent to have undergone a merger, when in fact there has been a chain shift. For example, an American may hear an Irish person use pronunciations like [bɑɹn] for born, [fɑɹm] for form, and [kɑɹd] for cord and incorrectly conclude that Hiberno-English has undergone the card-cord merger. In fact, there is no merger in Hiberno-English: the words barn, farm, and card are pronounced [bæɹn, fæɹm, kæɹd].
When phonemic changes occur differently in the standard language and in dialects, the dialect pronunciation is often stigmatized. This can result in hypercorrection, when the dialect speakers attempt to imitate the standard language, but overshoot, as with the foot-strut split, where failing to make the split is stigmatized in Northern England, and speakers of non-splitting accents often try to introduce it into their speech, sometimes resulting in hypercorrections such as pronouncing pudding /pʌdɪŋ/.
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