Deck of cards developed for research in extrasensory perception (ESP). The cards were designed by Karl E. Zener, a colleague of Joseph Banks Rhine at Duke University’s Department of Psychology in the 1930s. Rhine had developed ESP tests using cards but ran into a number of methodological problems. He asked Zener, who had done special research in the psychology of perception, to design a set of cards whose symbols were more easily distinguished than the already existing alternatives.
The resultant Zener cards pictured five symbols: a circle, a square, wavy lines, a triangle, and a cross. The complete deck had 25 cards, each symbol appearing on 5 cards. There were no color differences as in ordinary playing cards. The number of cards and symbols made statistical analysis easier. They have been widely used in ESP tests ever since their introduction.