Card marking refers to the act of changing the appearance of specific cards in a deck. This may be done with physical deformation (bending, scarring, etc.) or via ink/color markings on the back of the cards. It is a type of cheating that allows a specific player to know an unrevealed card.

It is not necessary to mark all the cards. Marking only aces would give a player a significant advantage. If the cheater were to know all the cards and play an exceptional game they would be quickly discovered. However, by making limited use of the markings a cheater may just appear to be having a plausible good streak.

Physical marks are more common that ink markings as a cheater can easily mark any deck of cards while playing the game. That is, after several rounds with a fresh deck the cheater will have had ample opportunity to mark the cards. This occurs even unintentionally as players accidentally bend the cards. High quality plastic cards make this more difficult. Additionally a player may request that a new deck is brought out should they suspect the cards are marked.