A sound change, in historical linguistics, 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 some changes are described as "sporadic" and so 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 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 that are named after their authors like Grimm's Law, Grassmann's Law etc. Real-world sound changes often admit exceptions, but the expectation of their regularity or absence of exceptions is of great heuristic value