Design decisions in interactive geometry software

Software design is increasingly being recognized as an important factor in student learning when interacting with interactive geometry software (IGS). A categorization of the operations possible within an IGS is used to identify and analyse design decisions made in a number of current IGS programs: Cabri II Plus, Cabri 3D, Cinderella, GeoGebra and Geometer's Sketchpad. The analysis, in the context of exploring the area of a circle, is focused on construction, dragging, and alternative spatial and semantic views. A wide diversity of both design issues and individual design decisions was identified, illustrating both the scarcity of research in this area and a number of inevitable tensions, such as between functionality and complexity, and between static and dynamic geometry, related to which future research questions might be posed.