Zener cards were created by J. B. Rhine, who was a parapsychologist. They do not carry his name because he decided to honor his scientific peer and colleague named Karl Zener. The images chosen for the cards were decided upon by Zener, however, whose own profession was in perceptual psychological work. The men introduced them into the psychological community during the 1920s; the cards were devised so that they might have a way of measuring ESP in a statistical way.
There are five distinct images in every pack of Zener cards, which always contains five of each of the cards. There is a card with a circle, one with a plus sign or cross with lines of equal length and another which has wavy lines; the final two cards consist of a five-pointed star and a square, respectively. The star is an outlined shape, and the card with wavy lines always has three lines present. These different designs all appear on the front side of the cards.
The original cards were composed, unfortunately, of a thin paper through which they found test subjects could easily see. This made for very inaccurate testing, as many subjects were scoring very high results due to this transparent flaw. This motivated the scientists to make sure that future cards would be designed more carefully to avoid the same problem, allowing for more realistic statistical results.
Zener cards are used to test both clairvoyance and telepathy. The clairvoyance of subjects is determined by the tester selecting and noting each card of the deck without the test subject able to see it. Testers make use of different ways to safeguard these tests, such as having a third observer present to monitor conditions or using partitions to visually separate the tester and subject The subject must guess which card is seen by the tester, and the tester makes note of the answer as he proceeds through the deck. The level of clairvoyant ability is determined by the percentage of correct answers. For telepathic testing, it is the test subject who selects a card; his task is then to project the symbol on the card, using only his mental power, onto the mind of another person involved in the test.