An astonishingly powerful though little-known perspective illusion in which a pair of identical parallelograms representing the tops of two tables appear radically different (see illustration). The illusion was first presented by the US psychologist Roger N(ewland) Shepard (born 1929) in his book Mind Sights: Original Visual Illusions, Ambiguities, and Other Anomalies (1990, p. 48). Shepard commented that ‘any knowledge or understanding of the illusion we may gain at the intellectual level remains virtually powerless to diminish the magnitude of the illusion’ (p. 128). The illusion arises from our inability to avoid making three-dimensional interpretations of the drawings, according to which the identical parallelograms would represent very different shapes because of perspective foreshortening, and it is based on his less powerful parallelogram illusion, published in an edited book entitled Perceptual Organization (1981, pp. 297–9). Also called the tabletop illusion. Compare Ames room, corridor illusion, Müller-Lyer illusion, Ponzo illusion.
Turning the tables illusion. Almost unbelievably, the tabletop on the left is identical in shape and size to the one on the right, as can be confirmed by tracing either of the white parallelograms and placing the tracing over the other.