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 are similar tend to be grouped perceptually, so that (for example) the array OOxx tends to be perceived as two pairs. Also called equality grouping law. See also common fate, common region, connectedness grouping law, Prägnanz. Compare closure grouping law, good continuation, proximity grouping law.