This study examines code-switching in eighteenth-century interpersonal communication, focusing on the correspondence of musician and music historian Charles Burney. The paper builds on the authors' previous work on code-switching in the history of English texts, and draws on insights gained in research in interactional sociolinguistics. The results show variation in code-switching practices with regard to the relationship between the writer and recipient. Code-switching is more frequent in letters written between correspondents who have a close relationship. Switches can have a locally meaningful function, organising discourse, indicating stance, or indexing the writer’s identity. Switching can also be seen as a style which in itself indexes particular types of social memberships and relationships.