Playing With A Marked Deck
In a standard poker deck, the back sides of the cards are all identical so that it is impossible to tell the difference between cards by simply viewing their backs. This is critical for protecting the integrity of the game, and keeps the playing field level for all players. Some players do not want a fair game, but would rather gain an unfair tactical advantage over their opponents, even if it means cheating. One of the longest standing and most insidious types of cheating is the use of marked cards.
Players may mark the playing cards in any number of ways. Usually this is done by altering or defacing the back side of the card in a manner that is only perceptible to the cheater. Card backings often have complex patterns which can provide camouflage for illicit markings. The cheater may mark the cards by slightly crimping a corner, by scratching their backs with a ring or fingernail, or by marking their backs with tiny ink spots. In more sophisticated cheating operations, the markings are actually printed into the card backs, but you have to know where to look for them. These markings allow the cheater to read the contents of his opponents hands without being detected.