We formulated our 3D curve-type classification from discussions with artists, and observations of finished sketches. In sketches of layered models, such as Figures 2, and 10, a viewer’s 3D interpretation of a sketch is strongly dependent on the perceived underlying geometry. Considering artists try to unambiguously define 3D form while sketching [Nakayama and Shimojo 1992], we assume that a smooth 2D stroke represents a similarly smooth 3D curve, and that inflections and points of high curvature often partition strokes into segments, where stroke sections can belong to a different 3D curve type. Our set of four 3D curve-types (Figure 3), are as follows: