This chapter introduces the idea that language change is happened, as word meanings, pronunciations, and even our syntax changes. The study of historical linguistics as we know it became particularly prominent after 1786, when Sir William Jones gave a lecture at the Royal Asiatic Society, suggesting that the Germanic languages and the Romance languages descended from a common ancestor. Jones' and others' research lead to proposing that a number of languages of Europe and India can be traced to a common ancestor, a proto language called Proto Indo-European. Using comparative language data they can reconstruct the common ancestor language of those related languages. Words that are systematically similar across languages are called cognates. Cognates exhibit regular sound correspondences, or systematic similarities in pronunciation. The comparative method can also be used to determine whether different language families are related to each other, which would suggest they descended from a common parent or ancestor language.