A visual illusion in which a square superimposed on a fanlike pattern of diverging lines appears distorted (see illustration) into a trapezium (US trapezoid). It is closely related to the Ponzo illusion and the Orbison illusion. Also (ambiguously) called the Ehrenstein illusion. Compare Ehrenstein's brightness illusion. [Named after the German Gestalt psychologist Walter Ehrenstein (1899–1961) who first reported it in his doctoral dissertation in 1921 and published it in 1925]
Ehrenstein's square illusion. The rectangle is a perfect square.