Blackjack, also known as twenty-one, is one of the most popular casino card games in the world. Much of blackjack's popularity is due to the mix of chance with elements of skill and decision making, and the publicity that surrounds the practice of card counting, a skill with which players can turn the odds of the game in their favor by making betting decisions based on the values of the cards known to remain in the deck. Blackjack's precursor was vingt-et-un ("twenty-one"), which originated in French casinos around 1700, and did not offer the 3:2 bonus for a two-card 21.

When blackjack was first introduced in the United States it wasn't very popular, so gambling houses tried offering various bonus payouts to get the players to the tables. One such bonus was a 10-to-1 payout if the player's hand consisted of the ace of spades and a black Jack (either the Jack of clubs or the Jack of spades). This hand was called a "blackjack" and the name stuck even though the bonus payout was soon abolished.

Rules

Blackjack hands are scored by their point total. The hand with the highest total wins as long as it doesn't exceed 21; a hand with a higher total than 21 is said to bust. Cards 2 through 10 are worth their face value, and face cards (jack, queen, king) are also worth 10. An ace's value is 11 unless this would cause the player to bust, in which case it is worth 1. A hand in which an ace's value is counted as 11 is called a soft hand, because it cannot be busted if the player draws another card.

The goal of each player is to beat the dealer by having the higher, unbusted hand. Note that if the player busts he loses, even if the dealer also busts. If both the player and the dealer have the same point value, it is called a "push", and neither player nor dealer wins the hand. Each player has an independent game with the dealer, so it is possible for the dealer to lose to one player, but still beat the other players in the same round.

The minimum bet is printed on a sign on the table and varies from casino to casino, and even table to table. The most common minimum in the U.S. is $5. After initial bets are placed, the dealer deals the cards, either from one or two hand-held decks of cards, known as a "pitch" game, or more commonly from a shoe containing four or more decks. The dealer gives two cards to each player, including himself. One of the dealer's two cards is face-up so all the players can see it, and the other is face down. (The face-down card is known as the "hole card". In European blackjack, the hole card is not actually dealt until the players all play their hands.) The cards are dealt face up from a shoe, or face down if it is a pitch game.

A two-card hand of 21 (an ace plus a ten-value card) is called a "blackjack" or a "natural", and is an automatic winner. A player with a natural is usually paid 3:2 on his bet. In 2003 some casinos started paying only 6:5 on blackjacks - although this reduced payout has generally been restricted to single-deck games where card counting would otherwise be a more viable strategy, the move was decried by longtime blackjack players.

The play goes as follows:

• If the dealer has blackjack and the player doesn't, the player automatically loses.

• If the player has blackjack and the dealer doesn't, the player automatically wins.

• If both the player and dealer have blackjack then it's a push.

• If neither side has blackjack, then each player plays out his hand, one at a time.

• When all the players have finished the dealer plays his hand.

The player's options for playing his or her hand are:

• Hit: Take another card.

• Stand: Take no more cards.

• Double down: Double the wager, take exactly one more card, and then stand.

• Split: Double the wager and have each card be the first card in a new hand. This option is available only when both cards have the same value.

• Surrender: Forfeit half the bet and give up the hand. Surrender was common during the early- and mid-20th century, but is no longer offered at most casinos.

The player's turn is over after deciding to stand, doubling down to take a single card, or busting. If the player busts, he or she loses the bet even if the dealer goes on to bust as well.

After all the players have finished making their decisions, the dealer then reveals his or her hidden hole card and plays the hand. House rules say that the dealer must hit until he or she has at least 17, regardless of what the players have. In most casinos a dealer must also hit a soft 17 (such as an ace and a 6). The felt of the table will indicate whether or not the house hits or stands on a soft 17.

If the dealer busts then all remaining players win. Bets are normally paid out at the odds of 1:1.

Some common rules variations include:

• one card split aces: one card is dealt on each ace, player's turn is over.

• early surrender: player has the option to surrender before dealer checks for Blackjack.

• late surrender: player has the option to surrender after dealer checks for Blackjack.

• double-down restrictions: double-down allowed only on certain combinations.

• dealer hits a soft seventeen (ace-six, which can play as seven or seventeen)

• European No-Hole-Card Rule: the dealer receives only one card, dealt face-up, and does not receive a second card (and thus does not check for blackjack) until players have acted. This means players lose not only their original bet, but also any additional money invested from splitting and doubling down.

There are more than a few blackjack variations which can be found in the casinos, each has its own set of rules, strategies and odds. It is advised to take a look at the rules of the specific variation before playing.

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Insurance

If the dealer's upcard is an Ace, the player is offered the option of taking Insurance before the dealer checks his 'hole card'.

The player who wishes to take Insurance can bet an amount up to half his original bet. The Insurance bet is placed separately on a special portion of the table, which usually carries the words "Insurance Pays 2:1". The player who is taking Insurance is betting that the dealer's 'hole card' is a 10-value card, i.e. a 10, a Jack, a Queen or a King. Because the dealer's upcard is an Ace, this means that the player who takes Insurance is essentially betting that the dealer was dealt a natural, i.e. a two-card 21 (a blackjack), and this bet by the player pays off 2:1 if it wins.

Example: The player bets $10, the cards are dealt, the player's hand is 19, and the dealer shows an Ace. The player takes Insurance by betting an additional amount of $5. The dealer checks her hole card and sees that it's a 10-valued card. The player loses his $10 bet on his blackjack hand, but he wins the insurance bet, so the player gets 2:1 on his $5 Insurance wager and receives $10 (on top of the $5 which is returned to him). Note that the player came out even on that round (i.e. did not lose any money).

Conversely, a player may win his original bet and lose his Insurance bet. Let's say we have the same situation as above except this time the dealer's hole card is not a ten, but rather a seven. In this case the player instantly loses his $5 Insurance wager. (All Insurance wagers are settled as soon as the dealer turns over her 'hole card', before all else.) But the player wins his $10 bet. Note that the player made a net profit on that round.

Of course, a player may lose both his original bet and his Insurance bet.

Insurance is a bad bet for the player who has no direct knowledge nor estimation (e.g. through card counting) of the dealer's 'hole card' because Insurance has a negative expected value for the player. Even for the player who has been dealt a natural (a two-card 21) it is unwise to take Insurance. In such a case, the dealer usually asks the player "Even money?" This means that instead of 3:2, the player with the natural accepts to be paid off at 2:2. Thus it is exactly the same thing as buying Insurance, losing the Insurance bet and getting paid 3:2 on the natural. (If the player with the natural refuses the offer of "even money", and the dealer turns over his hole card to make a natural (a blackjack), it is a tie and the player's bet is returned to him.)

In casinos where a hole card is dealt, a dealer who is showing a card with a value of Ace or 10 may slide the corner of his or her facedown card over a small mirror or electronic sensor on the tabletop in order to check whether he has a natural. This practice minimises the risk of inadvertently revealing the hole card, which would give the sharp-eyed player a considerable advantage.

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Basic strategy

As in all casino games, the house has a statistical advantage over the players that will play itself out in the long run. But because blackjack, unlike other games, has an element of player choice, players can actually reduce the casino advantage to a small percentage by playing what is known as basic strategy. This strategy determines when to hit and when to stand, and also determines when doubling down or splitting is the correct action. Basic strategy is based on the player's point total and the dealer's visible card. There are slight variations in basic strategy depending on the exact house rules and the number of decks used. Under the most favorable conditions (single deck, downtown Las Vegas rules), the house advantage over a basic strategy player can be as low as 0.16%. Indeed, casinos offering special rules like surrender and double-after-split may actually be offering a positive expectation to basic strategy players; they are counting on players making mistakes to make money.

The following rules are beneficial to the player:

1. Doubles are permitted on any two-card hand except a blackjack.

2. Doubles are permitted after splitting.

3. Early surrender; the ability to forfeit half your wager against a face or ace before the dealer checks for blackjack.

4. Normal (aka "late") surrender.

5. Resplitting Aces.

6. Drawing more than one card against a split Ace.

7. Five or more cards with the total still no more than 21 as an automatic win (a "Charlie")

The following rules are detrimental to the player:

1. Less than 3:2 payout on blackjacks (6:5 and even 1:1 payouts have become common, especially in single-deck games, in Las Vegas since about 2003)

2. Dealer hits on soft seventeen (ace, six)

3. Splitting a maximum of once (to two hands)

4. Double down restricted to certain totals, such as 9-11 or 10,11

5. Aces may not be resplit

6. No-Peek (European) blackjack—player loses splits and doubles to a dealer blackjack

7. Player losing ties

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Basic strategy tables

The above is a basic strategy table for the most common 6- to 8-deck, Las Vegas Strip rules. Specifically: dealer hits on soft 17, double after split allowed, multiple split aces, one card to split aces, blackjack pays 3:2, and (optionally) late surrender.

Key:

S = Stand

H = Hit

D = Double

SP = SPlit

Rh = suRrender if allowed, otherwise Hit

Rs = suRrender if allowed, otherwise Stand

Rsp = suRrender if allowed, otherwise SPlit

In some LV Strip casinos you may still be able to find the older version of the multi-deck shoe game, where dealer stands on soft 17; those are usually high minimum ($50 or more) tables. This version is much more advantageous to the player, but requires a slightly modified basic strategy table.

Interactive strategy tables for each possible card-distribution in the shoe can be generated using a JavaScript based blackjack calculator

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Shuffle tracking

There are techniques other than card counting that can swing the advantage of casino 21 towards the player, at least in theory. (It must be noted, however, that almost all of these techniques are based on the value of the cards to the player and the casino, as originally conceived by Edward O. Thorp.) One such technique, mainly applicable in multi-deck games (aka shoes), involves tracking groups of cards (aka slugs, clumps, packs) during the play of the shoe, following them through the shuffle and then playing and betting accordingly when those cards come into play from the new shoe. This technique, which is admittedly much more difficult than straight card counting and requires excellent eyesight and powers of visual estimation, has the additional benefit of fooling the casino people who are monitoring the player's actions and the count, since the shuffle tracker could be, at times, betting and/or playing opposite to how a straightforward card counter would.

Arnold Snyder's articles in Blackjack Forum magazine were the first to bring Shuffle Tracking to the general public.

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Variants

Spanish 21 provides players with many liberal blackjack rules, such as doubling down any number of cards (with the option to 'rescue', or surrender only one wager to the house), payout bonuses for five or more card 21's, 6-7-8 21's, 7-7-7 21's, late surrender, and player blackjacks always winning and player 21's always winning, at the cost of having no 10 cards in the deck (though there are jacks, queens, and kings). With correct basic strategy, a Spanish 21 game has a lower house edge than a comparable blackjack game. Another casino game similar to blackjack is Pontoon.

Certain rules changes are employed to create new variant games. These changes, while attracting the novice player, actually increase the house edge in these games. Double Exposure Blackjack is a variant in which the dealer's cards are both face-up. This game increases house edge by paying even-money on blackjacks and players losing ties. Double Attack Blackjack has very liberal blackjack rules and the option of increasing one's wager after seeing the dealer's up card. This game is dealt from a Spanish shoe, and blackjacks only pay even money.

Chinese Blackjack is played by many in Asia, having no splitting of cards, but with other card combination regulations.