A shared representation is crucial to effective, natural man-machine interaction. Unfortunately, little is known about peoples mental representation of shape; this prevents construction of truly effective computer-supported 3-D design media. How do people represent shape? We observe that design is typically an iterative process, starting with sketching and proceeding through detailing. Thus several different representations are required to support the design process. We investigate peoples naive, preattentive representation of shape to understand what sort of representation would best support the initial sketching stage of design. We then use our conclusions to build and evaluate a multi-representation 3-D CAD tool, and demonstrate enhanced performance.