How does card counting affect the odds in blackjack?

In the context of blackjack, card counting is a skill-based method used to combat the house edge by tracking the ratio of high to low cards remaining in the deck.1

Contrary to the assumption that all betting systems can overcome house advantages, card counting is effective because a deck with a high proportion of remaining high cards increases the player's odds against the house. The strategy relies on the player placing small bets when the deck composition is unfavorable and increasing their wagers when a favorable proportion of high cards is identified.1

Because this technique allows skilled players to gain a mathematical advantage, many casinos have implemented countermeasures to protect their interests, such as using multiple decks and reshuffling frequently to minimize changes in card distribution. While some players have historically used computers to track cards, the use of such electronic aids is prohibited in casinos. It is important to note that while card counting is a recognized strategy in blackjack, it is distinct from ineffective betting systems used in games like roulette, where the house retains an insurmountable advantage regardless of the betting pattern.12