The final specialization within linguistics that we will cover is historical linguistics, the study of how and why languages change and the tracing of the consequences of such changes. Languages are subject to many forces, which cause modifications to phonology, morphology, vocabulary, and social use. These changes are usually small and unappreciable, since people are very conservative about their speech. However, over the course of hundreds and thousands of years, small changes can completely transform language content and structure. Thus a modern English speaker finds Old English of a thousand years ago as unintelligible as a foreign language. Moreover, as languages change, speakers who live in different communities speak increasingly divergent forms of the language, until the dialects evolve into complete separate languages, as Latin gave birth to the Romance languages: French, Spanish, Italian, Portuguese and Romanian.
The investigation of language change is a very complex undertaking, since languages change too slowly to provide much immediate evidence of the process whereby new forms are adopted. There are some written records of ancient languages, but these exist in only a few traditions. Moreover, writing serves as an incomplete representation of the speech behaviour that provides the actual linguistic data. The most consistent method with which linguists can reconstruct archaic languages and trace their relationships to modern versions is to compare current languages that bear evidence of common origins, the basis of the field of comparative linguistics.
The main technique of comparative historical reconstruction is to identify cognates, words similar in both form and meaning, from several languages under the assumption that such similarities reflect common roots of the words and languages in question. Languages that have the highest percentage of cognates are the most closely related and therefore were the most recent to diverge from a common origin language, called a proto-language. Languages with fewer similarities are less closely related and suggest a more distant common ancestor. Languages that have no cognates are considered to be unrelated. Historical relationships among languages are also indicated by whether or not their cognates sound similar or have taken on very different phonological forms.