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 backward from known languages to the properties of earlier, unattested languages and hypothesize sound changes that may have taken place in them. Lexical changes The study of lexical changes forms the diachronic portion of the science of onomasiology. The ongoing influx of new words into the English language (for example) helps make it a rich field for investigation into language change, despite the difficulty of defining precisely and accurately the vocabulary available to speakers of English. Throughout its history English has not only borrowed words from other languages but has re-combined and recycled them to create new meanings, whilst losing some old words. Dictionary-writers try to keep track of the changes in languages by recording (and, ideally, dating) the appearance in a language of new words, or of new usages for existing words. By the same token, they may tag some words eventually as "archaic" or "obsolete". Spelling changes Standardisation of spelling originated centuries ago. Differences in spelling often catch the eye of a reader of a text from a previous century. The pre-print era had fewer literate people: languages lacked fixed systems of orthography, and the handwritten manuscripts that survive often show words spelled according to regional pronunciation and to personal preference. Semantic changes Semantic changes are shifts in the meanings of existing words. Basic types of semantic change include: pejoration, in which a term's connotations become more negative amelioration, in which a term's connotations become more positive broadening, in which a term acquires additional potential uses narrowing, in which a term's potential uses are restricted After a word enters a language, its meaning can change as through a shift in the valence of its connotations. As an example, when "villain" entered English it meant 'peasant' or 'farmhand', but acquired the connotation 'low-born' or 'scoundrel', and today only the negative use survives. Thus 'villain' has