Abstract
Diagrams are widely used as abstract models of systems. The act of creating the initial diagram is a common problem solving strategy. Most diagrams are conceived and created with an old-fashioned pen on a piece of blank paper or a whiteboard. Current widget based tools, in contrast, are unsuitable for early design support because of the constraints they place on the user. Sketch diagramming tools provide the unrestrained interface of paper and the document preparation support of computers. However there are many outstanding challenges before we can provide really good sketch tools for tasks such as diagramming.