communicate visual ideas and can be used to present and reconsider drafts and concepts in product design. This paper introduces a real-time illustration technique that sketches the design and spatial assembly of CSG models. The illustration technique generates a graphical decomposition of the CSG model into disjunctive layers to extract 1) the perceptually important edges that outline the model?s outer and inner features and 2) the surface shading of the outer and inner faces. Then, the technique applies uncertainty to these layers to simulate a sketchy effect. Finally, the technique composes the sketched layers in depth-sorted order while ensuring a correct depth behavior in the frame buffer. Because the sketchy illustrations are frame-to-frame coherent, the technique can be used as a tool for interactive presentation and reconsideration of the design and spatial assembly of CSG models.