Counting techniques
Blackjack has dozens of proprietary counting systems. But there are only a few basic principles on which they are built. Let's arrange them from simple to complex.

Ace-Five
Ace-Five or ace-five is the easiest card counting technique in blackjack. It only counts aces and fives.

The essence of the system:

The countdown starts from zero.
Every five in the hand is +1, ace is -1.
The higher the number, the higher the chances of winning.

Benefits:

Does not require changes to the course of the game.
Easy to learn even for a beginner.
Disadvantages:

Doesn't give a big advantage in the game.
There are two modifications of the Ace-Five system. They use the same principle of counting, but differ in the way they change the size of the bet:

It doubles upon reaching a certain limit, for example, +5.
The amount corresponds to the specific values ​​of the account.
Plus or minus
It is also easy to count using the Plus-Minus system. This does not require a calculator - only attentiveness and quick reactions. By the way, it was this technique that was used in the film "Twenty One", based on real events.

The essence of the system:

Cards are divided into “small” (from two to six), “neutral” (from seven to nine) and “large” (from ten to ace) and have values ​​+1, 0 and -1, respectively.
The higher the score, the more favorable the situation for the player.

Benefits:

Simple.
Allows you to profitably change the strategy depending on the account.
Disadvantages:

Requires attentiveness, timely and correct strategy changes.
Inaccurate - the values ​​of the cards do not correspond to the real mathematical expectation.
For example, the exit of the fives from the game is more favorable than the two's, but according to the "Plus-minus" system, the score is the same. This omission was tried to be eliminated in improved versions with a large spread of values, but such techniques are more difficult to apply.

Hi-Opt
At the time of its creation, Hi-Opt had no analogues among other techniques. The authors used an unusual approach - they took into account not only the mathematical expectation, but also the caution of the game strategy.

The essence of the system:

Only threes, fours, fives, sixes (favorable) and tens (unfavorable) are counted.
The rest of the cards are not counted.
The fundamental difference between Hi-Opt and other techniques is that aces are ignored. The creators of the technique found this to be correct in terms of changing the course of the game.

Benefits:

Simplicity.
Suitable for cautious players.
Disadvantages:

You cannot use an aggressive strategy.
In theory, counting aces is effective, but in practice it can get in the way, especially with a lack of playing experience. Therefore, the Hi-Opt strategy is well suited for beginners.

Halves
"Halves" can be considered an improved version of "Plus or Minus". This technique is quite difficult and requires good numeracy skills.

The essence of the system:

The cards are assigned a value proportional to the change in the mathematical expectation of the game after they are released.
The higher the score, the more chances you have to win.

Benefits:

High accuracy.
Disadvantages:

Complexity.
Counting errors negate efficiency.
The "Half" technique in 1975 in his book "Professional Blackjack" was described by American player Stanford Wong. In the original version, the values ​​of the cards are half as much (from -1 to +1.5).

Thorp system
The Thorp system is one of the most complex techniques and is rather of theoretical interest.

The essence of the system:

The values ​​of the cards accurately reflect the change in mathematical expectation.
Jacks, queens and kings do not count towards the score.

Benefits:

Accurate.
Disadvantages:

It is very difficult to keep score "in your head"
Not all cards are taken into account.
The author of the system, Edward Thorpe, is not only a blackjack player and creator of the best-selling Beat the Dealer, but also a professor of mathematics. Therefore, he probably managed to put his own technique into practice.

A common misconception is that it takes several months to learn how to count cards. In fact, simple techniques can be mastered much faster, even without phenomenal memory and mathematical talents. After a few weeks of regular practice, the counting will take place "automatically", only attentiveness is required from the player.

How casinos are fighting card counting
Counting increases the chances of winning, which is why players who count cards are usually denied access to the casino. Observers are watching this, but it is, of course, impossible to calculate everyone. Then the owners of the gambling business take measures that reduce the efficiency of any system:

It is forbidden to raise rates.
Increase the number of decks in a shoe.
Shuffle machines are used.

Is it possible to count cards in an online casino
In an online game, the shoes are updated after each hand, so the counting is meaningless. You can count only in Live-casinos, where the dealer is a live person, and the cards are shuffled after several hands.

The ability to increase your chances of winning by counting cards is one of the reasons for the popularity of blackjack. Even simple techniques that provide small benefits make the game more interesting and profitable.