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When people say they are going to count cards, they are referring to a strategy in which cards are tracked in order to give a slight statistical advantage in a card game. Most commonly people count cards in blackjack, although it can also be used in games such as spades, hearts, or bridge.
The reason it is helpful to count cards in blackjack is that there are certain stackings of the deck that are helpful for the dealer, and certain stackings that are helpful for the player. A deck in which a large number of high-value cards, Ten, Jack, Queen, King, and Ace, remain, and a low number of low-value cards remain offers more opportunities for the player to get a blackjack, doubling-down has an increased chance of success, and the dealer is likely to bust. For the opposite reasons, a deck with few high-value cards left is beneficial for the dealer.
To count cards, you don’t actually track every individual card in a deck. This is a common myth, but would be largely impossible for all but a savant, and is largely unnecessary. Instead, to count cards people assign a score to each type of card, and then keep a running tally of that score.
At its most basic, this is known as the Hi-Lo system, and can be undertaken by anyone with rudimentary tracking and addition skills. The Hi-Lo system to count cards assigns all cards with a face value of One to Six with a value of +1, cards with a face value of Seven to Nine are given a value of 0, and cards with a face value of Ten through Ace are given a value of -1. As low value cards are played, the tally increases, meaning there is a higher ratio of high-value cards remaining. As high value cards are played, the tally decreases, meaning there is a higher ratio of low-value cards remaining. The higher the tally, the better the odds of doubling down successfully or hitting a blackjack. The lower the tally, the more the odds are stacked against the player.
There are other systems to count cards which get substantially more complex than the Hi-Lo system. These assign different values to different cards, and in some cases involve keeping a side tally of certain cards, most notably Aces. This is because there are circumstances where knowing how many Aces are remaining helps with figuring out betting odds.
Some players who count cards use a technique commonly known as wonging. A wonging player will wait to the side of a table, not joining in the game, and instead just watching to count cards. They’ll make the count until the number is sufficiently high that their playing odds are strong. Then they’ll sit in and play at a strong table, playing until the count becomes low enough that their betting is no longer worth it, at which point they’ll leave the table.
The profit margin in blackjack when a player counts cards is fairly small, but can be a steady income if done successfully. Blackjack without card counting generally offers the house roughly a 0.5% advantage. With card counting, in a standard six-deck game, the player gets up to about a 1% advantage.
And from what my blackjack-playing friends tell me, be subtle about your card counting talent. Casinos do not like people who are too good at it. They figure there's going to be some card counting in the course of play -- anyone with common sense can figure out when a lot of low cards, then high cards are bound to come up.
So people who are proficient card counters should probably spread their gifts around a bit, in order to keep from being banned at a casino for winning too much, in the casino's opinion.