The article is concerned with a central contribution of designing to information visualization in the digital humanities. The activity is characterized as one of externalization, instantiation in visible or tangible form of ideas that might otherwise be internal to the minds of the designer and other participants. A spectrum of different interpretations of this process in the existing literature is discussed, focusing on early attempts to theorize designing as a means of creating new knowledge, and especially the ways in which that new knowledge contributes to defining or redefining the question or problem to be addressed. The arguments are illustrated with recent practical examples from the authors’ own work in designing with a range of cultural organizations, museums and archives. The article concludes with reflections on how projects may best benefit from this work of design, empowering the designer as a co-researcher, alongside the historian, curator or other humanities scholar.