In historical linguistics, a sound change is a change in the pronunciation of a language. A sound change can involve the replacement of one speech sound (or, more generally, one phonetic feature value) by a different one (called phonetic change) or a more general change to the speech sounds that exist (phonological change), such as the merger of two sounds or the creation of a new sound. A sound change can eliminate the affected sound, or a new sound can be added. Sound changes can be environmentally conditioned if the change occurs in only some sound environments, and not others. The term "sound change" refers to diachronic changes, which occur in a language's sound system. On the other hand, "alternation" refers to changes that happen synchronically (within the language of an individual speaker, depending on the neighbouring sounds) and do not change the language's underlying system (for example, the -s in the English plural can be pronounced differently depending on the preceding sound, as in bet[s], bed[z], which is a form of alternation, rather than sound change). Since "sound change" can refer to the historical introduction of an alternation (such as postvocalic /k/ in the Tuscan dialect, which was once [k] as in di [k]arlo 'of Carlo' but is now [h] di [h]arlo and alternates with [k] in other positions: con [k]arlo 'with Carlo'), that label is inherently imprecise and must often be clarified as referring to either phonemic change or restructuring. Research on sound change is usually conducted under 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 non-phonological factors like the meaning of the words that are affected. Apparent exceptions to regular change can occur because of dialect borrowing, grammatical analogy, or other causes known and unknown; and changes are described as "sporadic" when they affect only one or a few particular words, without any apparent regularity. The Neogrammarian linguists of the 19th century introduced the term sound law to refer to formulations of regular change, perhaps in imitation of the laws of physics, and the term "law" is still used in referring to specific sound changes that are named after their authors like Grimm's law and Grassmann's law. Real-world sound laws often admit exceptions, but the expectation of their regularity or absence of exceptions is of great heuristic value by allowing historical linguists to define the notion of regular correspondence by the comparative method. Each sound change is limited in space and time and so it functions in a limited area (within certain dialects) and for a limited period of time. For those and other reasons, the term "sound law" has been criticized for implying a universality that is unrealistic for sound change. A sound change that affects the phonological system or the number or distribution of its phonemes is a phonological change.
Principles The following statements are used as heuristics in formulating sound changes as understood within the Neogrammarian model. However, for modern linguistics, they are not taken as inviolable precepts but are seen as guidelines. Sound change has no memory: Sound change does not discriminate between the sources of a sound. If a previous sound change causes X,Y > Y (features X and Y merge as Y), a new one cannot affect only an original X. Sound change ignores grammar: A sound change can have only phonological constraints, like X > Z in unstressed syllables. For example, it cannot affect only adjectives. The only exception is that a sound change may recognise word boundaries, even when they are unindicated by prosodic clues. Also, sound changes may be regularized in inflectional paradigms (such as verbal inflection), when it is no longer phonological but morphological in nature. Sound change is exceptionless: If a sound change can happen at a place, it will affect all sounds that meet the criteria for change. Apparent exceptions are possible because of analogy and other regularization processes, another sound change, or an unrecognized conditioning factor. That is the traditional view expressed by the Neogrammarians. In the past decades, however, this has been challenged by the theory of lexical diffusion, which argues that sound change need not necessarily affect all possible words. However, when a sound change is initiated, it often eventually expands to the whole lexicon. For example, the Spanish fronting of the Vulgar Latin [g] (voiced velar stop) before [i e ɛ] seems to have reached every possible word. By contrast, the voicing of word-initial Latin [k] to [g] occurred in colaphus > golpe and cattus > gato but not in canna > caña. Sound change is inevitable: All languages vary from place to place and time to time, and neither writing nor media prevents that change.
Formal notation
A statement of the form
A > B is to be read as "Sound A changes into (or is replaced by, is reflected as, etc.) sound B". Therefore, A belongs to an older stage of the language in question, and B belongs to a more recent stage. The symbol ">" can be reversed, B < A, which also means that the (more recent) B derives from the (older) A":
POc. *t > Rot. f means that "Proto-Oceanic (POc.) *t is reflected as [f] in Rotuman (Rot.)". The two sides of such a statement indicate only the start and the end of the change, but additional intermediate stages may have occurred. The example above is actually a compressed account of a sequence of changes: *[t] first changed to [θ] (like the initial consonant of English thin), which has since yielded [f] and can be represented more fully:
t > θ > f Unless a change operates unconditionally (in all environments), the context in which it applies must be specified:
A > B /X__Y = "A changes to B when it is preceded by X and followed by Y." For example:
It. b > v /[vowel]__[vowel], which can be simplified to just It. b > v /V__V (in which the V stands for any vowel) = "Intervocalic [b] (inherited from Latin) became [v] in Italian" (such as in caballum, dēbet > cavallo 'horse', deve 'owe (3rd pers. sing.)' Here is a second example:
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