Editing Language change (section)

===Phonetic and phonological changes===
{{Main|Sound change|Phonological change}}

[[Sound change]]—i.e., change in the pronunciation of [[phoneme]]s—can lead to [[phonological change]] (i.e., change in the relationships between phonemes within the structure of a language). For instance, if the pronunciation of one phoneme changes to become identical to that of another phoneme, the two original phonemes can merge into a single phoneme, reducing the total number of phonemes the language contains.

Determining the exact course of sound change in historical languages can pose difficulties, since the technology of [[Sound recording and reproduction|sound recording]] dates only from the 19th century, and thus sound changes before that time must be inferred from written texts. The [[Orthography|orthographical]] practices of historical writers provide the main (indirect) evidence of how language sounds have changed over the centuries. Poetic devices such as rhyme and rhythm can also provide clues to earlier phonetic and phonological patterns.

A principal axiom of historical linguistics, established by the linguists of the [[Neogrammarian]] school of thought in the 19th century, is that sound change is said to be "regular"—i.e., a given sound change simultaneously affects all words in which the relevant set of phonemes appears, rather than each word's pronunciation changing independently of each other. The degree to which the Neogrammarian hypothesis is an accurate description of how sound change takes place, rather than a useful approximation, is controversial; but it has proven extremely valuable to historical linguistics as a [[heuristic]], and enabled the development of methodologies of [[comparative reconstruction]] and [[internal reconstruction]] that allow linguists to extrapolate backwards from known languages to the properties of earlier, un[[attested language]]s and hypothesize sound changes that may have taken place in them.