Magicians Using Marked Cards
A marked deck has a subtle mark on the decorative side of the cards. Usually the mark is placed in a certain position to indicate the number of the card, with the color or shape of the mark indicating suit. The magician shows the card to the audience and reads off the number and suit of the card without seeing it, indicating x-ray vision. This principle is derived from similar card marking techniques used to cheat at gambling. The cards of a marked deck may be marked in a number of ways. They may be manufactured with subtle variations in the back pattern. They may be marked by the user with tiny punctures or dots, or a barely detectable change of the tint of the cardstock or pattern. They may even be marked live, while in use either with a sharpened finger nail or a tool (often a ring known as a poker ring) made specifically for that purpose.