The term “marked cards” refers to a deck of playing cards that have been altered in some way so that it becomes easier for someone to identify them. They are often used to cheat at card games, and honest players should be aware of how they could be marked in order to avoid being cheated.
In the past, cards were marked by bending them and crimping to make the subtle markings indistinct to spectators. People then developed more sophisticated methods of marking cards, such as pinprick bumps or cut-out work. Some of these methods require special gadgets to detect, while others require high levels of skill.
Another way to hide any visible markings is to apply a chemical called “juice dust” or “daub”. This can be done while playing. Lastly, there are video-luminous marks that can only be detected through special sunglasses and cameras. This technique was developed by magicians who use it to read their opponent’s cards. However, some professional gamblers have also used it to gain an advantage.
The best card magic marking system is one that is undetectable to the spectators but easy to read by a magician. Tiny marks can be difficult to read and cause the magicians to make mistakes. For example, a small mark in the corner of a card may indicate that it is a mountain, Loxodon Hierarch, or Lightning Helix–giving the player undue knowledge about the card’s power.