one, which then tend to diverge because of phonemic differentiation. Chain shifts 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 has triggered a fronting of , which in turn has triggered a lowering of , and so forth. Phonemic mergers If a phoneme moves in acoustic space, but its neighbors do not move in a chain shift, a phonemic merger may occur. In that case, a single phoneme results where an earlier stage of the language had two phonemes (that is also called phonetic neutralization). A well known example of a phonemic merger in American English is the cot–caught merger by which the vowel phonemes and (illustrated by the words cot and caught respectively) have merged into a single phoneme in some accents. Phonemic splits In a phonemic split, a phoneme at an earlier stage of the language is divided into two phonemes over time. Usually, it 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 originally had front rounded allophones before the vowel in a following syllable. When sound change caused the syllables containing to be lost, a phonemic split resulted, making distinct phonemes. It is sometimes difficult to determine whether a split or a merger has happened if one dialect has two phonemes corresponding to a single phoneme in another dialect; diachronic research is usually required to determine the dialect that is conservative and the one that is innovative. When phonemic change occurs differently in the standard language and in dialects, the dialect pronunciation is considered nonstandard and may be stigmatized. In descriptive linguistics, however, the question of which splits and mergers are prestigious and which are stigmatized is irrelevant. However, such stigmatization can lead to 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 . Occasionally, speakers of one accent may believe the speakers of another accent to have undergone a merger, when