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 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). 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.