A visual illusion in which the long diagonal of a parallelogram appears much longer than that of a smaller parallelogram contiguous to it, one of the large parallelogram's short sides forming one of the long sides of the smaller parallelogram and the two long diagonals being of equal length (see illustration). Also called the parallelogram illusion. [Named after the German Gestalt psychologist Friedrich Sander (1889–1971) by one of his students, who published it in 1926, although it had already been published by the US lighting engineer and psychologist Matthew Luckiesh (1883–1967) in his book Visual Illusions: Their Causes, Characteristics and Applications (1922, p. 58) and should arguably be called the Luckiesh illusion]
Sander parallelogram. The distances AX and AY are equal.