When playing blackjack, there are sophisticated systems that require you to count cards. These systems work only when the game is being dealt from a single deck, unless you know for sure that the decks in a shoe have not been shuffled.
If you know that for sure, you must be working for the house or cheating, and in either case you're asking for trouble.
If the casino where you're playing still deals from single decks, and if you can count cards without giving yourself away by making marks on a pad, or holding up your fingers, you can improve your betting.
The first cards to count, naturally, are the 10-point cards: 10s, jacks, queens, and kings.
These cards are the basic building blocks of blackjack, the ones that can make or break you, depending on when you get them.
If you are sure that a lot of 10-point cards are still out toward the end of the deck, you might want to increase your bet, if that's allowed. But be sure. A casual "sense" of the cards is not good enough.
Either keep track of the cards in a serious way, or forget it, and regard the odds as the same throughout the deal. Since there are sixteen 10-point cards in a deck, counting them on your fingers won't do.
Some people who play systems have gimmick jewelry, for example, with 16 different stones. They can tell at a glance how many 10-point cards have been played by looking at which stone is turned up on the bracelet.
But you have to practice maneuvers like this you'll be as obvious as a kid with a new magic trick. Once the dealer recognizes that you're counting cards, he or she can shuffle the deck at any time, reducing all your efforts to nothing.
In the first place, avoid, if you can, blackjack games that are dealt from a shoe.
Next, avoid games where the cards are shuffled after every round, which is just as bad or worse. Under these two systems, nobody can count cards.
For the same reasons, and more, never play blackjack with a computerized dealer. Yes, you can be sure that the Darth Vader of the blackjack set doesn't cheat you, because it's a machine.
But your options of splitting pairs are much more limited than the entire human dealer's table, and you can't double-down at all.
Why anyone would prefer to play with this preprogrammed sucker trap is somewhat mysterious, but there are obviously those who are too inhibited to gamble with people looking on and think they'd do better facing a machine to take their money.
Two other traps exist which are not machines, or tricks by the casino, but tricks the player plays on themselves. These are two favorite folk systems, the "copy-the-dealer" and "no-bust'" systems.
Copying the dealer means you always hit a stiff hand of 16 or less, and stand at 17, just as the house rules require the dealer to do. "No-bust" is just the opposite - you never hit a stiff.
Many wouuld suggest to you to avoid using these "folk" systems. By relinquishing the freedom the player has to exercise their judgment, blackjack is changed from an interesting game of skill and luck into one as mechanical as baccarat, its casino cousin.