Card counting is a technique employed by so-called ‘advantage’ Blackjack players to reverse the house edge – which is less than 1% in any case – and give themselves a small mathematical edge, typically between 0.5% and 1.5%, over the game.
One of the most popular card counting techniques, known as the ‘high-low count’, assigns a value of -1 to aces, court cards and tens, which are considered favourable to the player, +1 to twos, threes, fours, fives and sixes, which are considered favourable to the dealer, and 0 to sevens, eights and nines, which are considered neutral. Fairly obviously, at the start of a shoe the so-called ‘running count’ is 0, so the card counter simply adds or subtracts the appropriate value as each cards is revealed. The final step is to divide the running count by the number of decks left in the shoe, or a rough approximation thereof, to provide the so-called ‘true count’. As the true count rises and falls, the card counter can raise and lower his bets, and adjust his playing strategy, accordingly.
As far as legality is concerned, card counting essentially involves just basic arithmetic and requires nothing more sophisticated than the human brain, so is perfectly legal. Of course, card counters cannot win every hand they play; it is only over the course of hundreds of hours playing and tens of thousands of hands that they can expect to make a profit. Even so, casinos take a dim view of card counting and, if they suspect that a player has an advantage over the game, even in the long-term, take steps to remove the advantage. This could simply involve shuffling the cards when a card counter raises his bet or, in certain jurisdictions, to stop playing Blackjack and/or leave the premises.