Zener Cards

Zener Cards (ゼナー・カード), also known as ESP Cards, are cards used in System Scans, most specifically, clairvoyance.

Appearance
There are five different Zener cards: a hollow circle (one curve), a Greek cross (two lines), three vertical wavy lines (or "waves"), a hollow square (four lines), and a hollow five-pointed star. There are 25 cards in a pack, five of each design.

Principles
The esper being tested must be able to determine what the contents of the card are correctly.

Background
Perceptual psychologist designed the cards in the early 1930s for experiments conducted with his colleague, parapsychologist.

In Toaru Majutsu no Index, the card's material was changed from vinyl to ABS resin, which was instituted by the United States of America in 1992 due to a trick that the fingerprints on the card made it possible to determine the card's contents.

Sisters Arc
In Tsukuyomi Komoe's supplementary lessons, she gives a lecture of the history of the ESP Card to an unenthusiastic Kamijou Touma.

Three Stories Arc
In the anime adaptation, cards are shown on Touma's coffee table during his attempt at finishing his summer homework.

Level Upper Arc
In the System Scan of the anime adaptation of Toaru Kagaku no Railgun, a clairvoyant or a telepath is being tested using the ESP Card.