Sound shift

Sound change includes any processes of language change that affect pronunciation (phonetic change) or sound system structures (phonological change). Sound change can consist of the replacement of one speech sound (or, more generally, one phonetic feature value) by another, the complete loss of the affected sound, or even the introduction of a new sound in a place where there had been none. Sound changes can be environmentally conditioned, meaning that the change only occurs in a defined sound environment, whereas in other environments the same speech sound is not affected by the change. The term "sound change" refers to diachronic changes—that is, irreversible changes in a language's sound system over time; "alternation", on the other hand, refers to changes that happen synchronically (i.e. within the language of an individual speaker, depending on the neighboring sounds) and which do not change the language's underlying system (for example, the -s in the English plural can be pronounced differently depending on what sound it follows; this is a form of alternation, rather than sound change). However, since "sound change" can refer to the historical introduction of an alternation (such as post-vocalic /k/ in Tuscan—once [k], but now [h])—the label is inherently imprecise and often must be clarified as referring to phonetic change or restructuring.

Research on sound change is usually conducted on the working assumption that it is regular, which means that it is expected to apply mechanically whenever its structural conditions are met, irrespective of any nonphonological factors (such as the meaning of the words affected). However, apparent exceptions to regular change can occur—due to dialect borrowing, grammatical analogy, or other causes, known and unknown—and some changes are described as "sporadic", meaning that they affect only one particular word or a few words, without any apparent regularity.

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