Designing for Change

in both research and practical efforts within a node-link model: nodes are the holders of content, and links are the means by which the content is given context. In this model, links are closely associated with navigation and mechanisms for traversal; they are a way to move from node to node, to keep readers focused on the current node or document, until they decide to move on to the next. We refer to this style of hypertext as document-centered hypertext. As systems and applications designers have gained more practical experience with hypertext, models of structure have grown more sophisticated, more expressive , and in many cases, more flexible than conventional node-link models. To address the needs of specialized applications, some models diverge entirely from standard notions of links; for example, Parunak introduces set-based hypertext for applications that involve taxo-nomic reasoning [21]. The expressiveness of hypertext models has been extended by adding types to nodes, links, and structures [5, 15, 20]. Finally, for the sake of flexibility, in some applications links are left implicit, computed dynamically by using text analysis methods or other heuristics for determining regular interconnec-tions [2, 6]. Thus we see systems and applications designers broadening the field, in some cases by theory, but also through practical experience. Spatial hypertext has grown out of just such an intertwining of theory-and experience-based extensions to hypertext concepts. Spatial hypertext has arisen through our experiences with applications that explore alternative structures for content and applications in which the domain structure is not well understood at the outset or changes during the course of a task. Many of these applications involve the collection, comprehension, ypertext, 1 in its most general sense, allows content to appear in different contexts. The immediate setting in which readers encounter a specific segment of material then changes from reading to reading or from reader to reader. Authors collect and structure materials to reflect their own understanding or in anticipation of readers' possible interests, needs, or ability to comprehend the substrate of interrelated content. 1 We use the term hypertext broadly, to cover both textual and multimedia content.