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) by another, the complete loss of the affected sound, or even the introduction of a new sound in a place where there previously was none. Sound changes can be environmentally conditioned, meaning that the change in question 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, or changes in a language's underlying sound system over time; "alternation", on the other hand, refers to surface changes that happen synchronically and 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).
Sound change is usually assumed to be regular, which means that it is expected to apply mechanically whenever its structural conditions are met, irrespective of any non-phonological factors (such as the meaning of the words affected). On the other hand, sound changes can sometimes be sporadic, affecting only one particular word or a few words, without any seeming regularity.
For regular sound changes, the term sound law is sometimes still used. This term was introduced by the Neogrammarian school in the 19th century and is commonly applied to some historically important sound changes, such as Grimm's law. While real-world sound changes often admit exceptions (for a variety of known reasons, and sometimes without one), the expectation of their regularity or "exceptionlessness" is of great heuristic value, since it allows historical linguists to define the notion of regular correspondence (see: comparative method).
Each sound change is limited in space and time. This means it functions within a specified area (within certain dialects) and during a specified period of time. For these (and other) reasons, some scholars avoid using the term "sound law" — reasoning that a law should not have spatial and temporal limitations — replacing the term with phonetic rule.
Sound change which affects the phonological system, in the number or distribution of its phonemes, is covered more fully at phonological change.
Read more about Sound Change: The Formal Notation of Sound Change, Principles of Sound Change, Terms For Changes in Pronunciation, Examples of Specific Historical Sound Changes
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