Blackjack is one of the most popular card games in the world. Its simple rules and strategic depth have fascinated players for generations. However, as in any game where money is at stake, unscrupulous people look for ways to tip the odds in their favor. Card marking is one such method used for fraud. In this article, we will delve into the underground world of card tagging, explore different methods, and offer ideas on how to detect and stop these dishonest practices.