Historical linguistics studies the history and evolution of languages through the comparative method. Often, the aim of historical linguistics is to classify languages in language families descending from a common ancestor. This involves comparison of elements in different languages to detect possible cognates in order to be able to reconstruct how different languages have changed over time. This also involves the study of etymology, the study of the history of single words. Historical linguistics is also called "diachronic linguistics" and is opposed to "synchronic linguistics" that study languages in a given moment in time without regarding its previous stages. In universities in the United States, the historic perspective is often out of fashion. Historical linguistics was among the first linguistic disciplines to emerge and was the most widely practiced form of linguistics in the late 19th century. The shift in focus to a synchronic perspective started with Saussure and became predominant in western linguistics with Noam Chomsky's emphasis on the study of the synchronic and universal aspects of language.