Zener cards were invented by parapsychologist J. B. Rhine as an easily statistically measurable, unambiguous way of testing for ESP according to scientific method. Rhine named them in honor of his colleague Karl Zener, a perceptual psychologist. Dr. Zener had selected the five designs that would appear on the cards. When Zener cards were first invented in the 1920s, they were shuffled by hand, but Rhine later switched to having a machine shuffle them.
There are 25 Zener cards in a pack, with 5 cards each of each design. The 5 designs on the fronts of the cards are a circle, a cross in the Greek cross form with each of the four lines being of equal length, a five-pointed star in outline form, a square, and a trio of vertical wavy lines (the "waves").
In order, the cards are: circle, plus, wavy lines, square, star - in the order of the number of lines used. (The star has five points)
When Zener cards were first used, they were made out of a fairly thin translucent white paper. Several subjects or groups of subjects scored very high in early years, but it was soon discovered that the subjects had often been able to see the symbols through the backs of the cards. They were then redesigned to make it impossible to see the designs through the backs under any conditions. A subsequent deck used a back with an illustration of a building on the Duke University campus - but that was a very bad choice, making the deck into a "one-way," which delighted the magicians, who promptly designed tricks that could be done with the Zener cards.
In the tests for clairvoyance, the person conducting a test picks up a card in a shuffled pack, looks at it to verify what symbol is on the card, and records the answer of the person being tested for ESP, who must correctly determine which of the five designs is on the card in question. The experimenter then continues until all the cards in the pack have been tested. A third person may be hired to oversee or videotape the experiment to make sure it is conducted fairly and that all cards are clearly unable to be seen by the subject by visual means. Physical separators may be used between the tester and the subject. Elaborate experiments with Zener cards, just as with other forms of testing for ESP, have been designed that find all ways of keeping the subject unable to see the cards or the face of the experimenter, finding ways for the actual cards and responses to be recorded while the two are in different rooms.
Being that there are five of each of the five symbols, the obvious score expected of someone guessing at chance would be 20% right. The more times a person is tested on a pack of 25 cards, the more statistically meaningful the score becomes. A person could easily get 32% or only 12% right by chance on a single trial, but if the subject is not psychic the scores for several runs are expected to average out to 20% over time. A person with budding ESP, or a small but existent amount of ESP -- knowledge of the outside world through something that cannot be explained by the five senses -- would maintain an average of significantly higher than 20% over time, averaging 38% or 51%, perhaps. A score of 100% would indicate a world-class psychic, a person who was perfectly clairvoyant. A long-term average of something significantly lower than what chance would indicate, for instance 8%, might be an indicator of negative ESP.
Zener cards may be used to measure the innate ESP of an individual, but they may also be used to indicate the average scores for groups of subjects being tested. Experiments that test hypotheses about whether ESP can be learned, its connections with hypnosis, its prevalence in different demographic groups, how nutrition affects psychic ability or other general trends may use a target group and a control group and compare the average scores for each group. In this case the degree of statistically significant difference between the averages for the target and control group as well as the general deviation from the 20% norm must be taken into account.
Although Zener cards are usually used to test for clairvoyance, they may also be used to test for telepathy. A subject will draw a card from the deck and will try to mentally project the image of whichever design is on that card onto the mind of another person. Here, the statistical tendency of the person who is supposed to report the telepathic signals to report a specific design must also be taken into account -- perhaps the person "listening" for the telepathic signals has a particular propensity to report squares, for example -- so therefore what percentage of the reported squares were actually cards with squares, what percentage of the reported crosses were actually cards with crosses, etc. must be calculated. Astronaut Edgar Mitchell famously used Zener cards in this fashion in an unofficial experiment during the Apollo 14 mission to the moon in 1971.
A demonstration with Zener cards successful to a statistically significant level would qualify for the James Randi million dollar challenge.