Blackjack is unique among casino games. Blackjack is commonly played with an eight deck shoe. As play continues, cards are removed from the shoe and placed in a discard pile. This creates a game history. Other casino games are random chance, such as roulette or craps, where each spin or roll is independent of the last play. But in blackjack a discarded card is removed from play and will not be seen again until the shoe is shuffled.

A blackjack player who knows how to count cards can take advantage of this game history. Within an eight deck shoe you know there are 32 aces. If 30 of them have been played and discarded, only 2 remain in the shoe. You can then adjust your play based on the remaining cards in the shoe. That is heart of every card counting system, knowing what cards remain in the shoe.