The Liber Lynne, a fifteenth-century manuscript in the archive of the City of London Corporation, is a puzzle. Catalogued among the City of London’s collections of written custom (formerly Guildhall MS Cust. 15), it is generally defined as a cartulary. In this article, I study the Liber Lynne as a book that was both about family and for family. Its chance survival, a consequence of its acquisition by the Hanseatic Steelyard in London before the end of the fifteenth century, offers an unusual opportunity to explore the concept of family in the medieval English town. I situate the Liber Lynne in a distinct place and time, and argue that the book is a distinctively urban manuscript, the outcome of urban interests, ambitions, and anxieties. It also reveals the persistent and ubiquitous presence of plague, which exposed the fragility and precarity of families, but helped to give them different shapes. These shapes, or structures, were fluid because of the mutable nature of ideas about family and its voluntaristic qualities. Family, the Liber Lynne suggests, was a choice and a practice.