It is quite commonplace for bilingual speakers to use two or more languages, dialects or varieties in the same conversation, without any apparent effort. The phenomenon, known as code-switching, has become a major focus of attention in linguistics. This concise and original study explores how, when and where code-switching occurs. Drawing on a diverse range of examples from medieval manuscripts to rap music, novels to advertisements, emails to political speeches, and above all everyday conversation, it argues that code-switching can only be properly understood if we study it from a variety of...