Above each geon is the geon's name.  Below each geon is a list of nonaccidental properties :  (1) straight or curved edges, (2) straight or curved axis, and (3) constant, expanded, or expanded and contracted sides. Certain properties of edges in a two-dimensional drawing are taken by the visual system as evidence that those edges in a three-dimensional world would contain the same properties.  So, for example, if a two-dimensional drawing contains a curved line, the visual system infers that the same smoothly curved feature would exist in a three-dimensional setting.  This inference made by the visual system demonstrates the principle of curvilinearity.  Properties such as curvilinearity have been termined nonaccidental (Witkin & Tenenbaum, 1983) because they would only rarely be produced by accidental alignments of viewpoint and object features.  An example of an acciental alignment is if a curved line fell on the retina in such a way that the curvature of the line exactly matched the curvature of the retina.  Here, the line would be perceived as straight instead of curved.  An accidental alightment of this sort is highly unlikely.  Thus, the visual system operates under the assumption that the image falling on the retina is not an accident of viewpoint.