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, 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, as in bet[s], bed[z]; 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] as in di [k]arlo 'of Carlo', but now [h] di [h]arlo, alternating with [k] in other positions: con [k]arlo 'with Carlo')—the label is inherently imprecise and often must be clarified as referring to phonemic 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 non-phonological 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.
The Neogrammarian linguists of the 19th century introduced the term "sound law" to refer to rules of regular change, perhaps in imitation of the laws of physics, and the term "law" is still used in referring to specific sound rules named after their authors, such as Grimm's Law, Grassmann's Law, etc. Real-world sound changes often admit exceptions; nevertheless, the expectation of their regularity or absence of exceptions 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 limited area (within certain dialects) and during a limited period of time. For these (and other) reasons, the term "sound law" has been criticized for implying a universality that is unrealistic with regard to sound change.
is to be read, "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, whereas B belongs to a more recent stage. The symbol ">" can be reversed, B < A, still meaning that the (more recent) B derives from the (older) A".
The two sides of such a statement indicate start and end points only, and do not imply that there are no additional intermediate stages. The example above is actually a compressed account of a sequence of changes; *t changed first into a voiceless dental fricative [θ] (like the initial consonant of English thin), which has yielded present-day [f]. This can be represented more fully as:
Unless a change operates unconditionally (in all environments), the context in which it applies must be specified:
The symbol "#" stands for a word boundary (initial or final). Thus the notation "/__#" means "word-finally", and "/#__" means "word-initially". For example:
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 rules; rather, they 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 only have phonological constraints, like X > Z in unstressed syllables. For example, it cannot only affect adjectives. The only exception to this is that a sound change may or may not recognise word boundaries, even when they are not indicated by prosodic clues. Also, sound changes may be regularized in inflectional paradigms (such as verbal inflection), in which case the change is no longer phonological but morphological in nature.
Sound change is exceptionless: if a sound change can happen at a place, it will. It affects all sounds that meet the criteria for change. Apparent exceptions are possible, due to analogy and other regularization processes, or another sound change, or an unrecognized conditioning factor. This is the traditional view, expressed by the Neogrammarians. In past decades it has been shown that sound change does not necessarily affect all the words that in principle it could. However, when a sound change is initiated, it often expands to the whole lexicon given enough time, though not always. For example, in Spanish the fronting of the Vulgar Latin [g] (voiced velar stop) before [i e ɛ] seems to have reached every possible word it could. By contrast, the voicing of word-initial Latin [k] to [g] occurred in colaphus > golpe and cattus > gato, but not in canna > caña. See also lexical diffusion.
Sound change is inevitable: All languages vary from place to place and time to time, and neither writing nor media prevent this change.
In historical linguistics, a number of traditional terms designate types of phonetic change, either by nature or result. A number of such types are often (or usually) sporadic, that is, more or less accidents that happen to a specific form. Others affect a whole phonological system. Sound changes that affect a whole phonological system are also classified according to how they affect the overall shape of the system; see phonological change.