One of the four original grouping laws of Gestalt psychology, formulated in 1923 by the German psychologist Max Wertheimer (1880–1943) to explain the organization of parts into wholes by the visual system. According to the law, elements that move in the same direction tend to be grouped perceptually, hence the symbol × is perceived as being composed of two straight lines rather than (for example) two wedges > and <, and the symbol Ø is perceived as a circle crossed by a straight line. A special case of this law for elements moving in time is called common fate. See also common region, connectedness grouping law, Prägnanz. Compare closure grouping law, proximity grouping law, similarity grouping law.