The chapter explains the basic principles of linguistic change from a sociolinguistic variationist perspective. It begins with an explanation of the inextricable relationship between linguistic variation and change, and proceeds to demonstrate how language change can be observed, investigated, and explained. Sociolinguists can document and analyse language change using either the real-time method or the apparent-time construct; these methods and their advantages and pitfalls are explained and exemplified.

Chapter 8 examines how languages change and what the past stages of languages may be. It distinguishes between diachronic linguistics, which looks at languages over time, and synchronic linguistics, which focuses on a particular stage. It also contrasts the concept of language as applied to a speech community (E-language) and language as the state of knowledge of an individual speaker (I-language). The chapter offers readers an understanding of changes that occur in languages' phonology, morphology, syntax, semantics and lexicon, thus linking this chapter to previous ones. It explains how linear descent can be established by finding regular correspondences between the forms in one stage and a previous stage, and that the changes that occur can be attributed to imperfect transmission between one individual and the next, and the results may then spread to the community. It contrasts this with non-linear descent, in which similar forms are not due to a common ancestor. It compares present languages to their past, and explores the reasons why languages change. The chapter also examines in detail how unattested languages are reconstructed by applying methods of comparative linguistics.