Research into collaborative document use often concentrates on how people share document content. However, studies of real-world document practices reveal that the structures by which document corpora are organised may also, themselves, be important sites of collaborative activity. Unfortunately, this poses a problem. When category structures are used to understand a set of documents, the manipulation of those structures can interfere with shared understanding and intelligibility of the document space. We show how this problem arises in real-world settings, using a case arising from some recent field work. We outline a solution to the customisation/intelligibility problem, and show how it has been implemented in a system for personal and workgroup document management.