Blackjack, also known as Twenty-one or Vingt-et-un , is the most widely played casino banking game in the world. Blackjack is a comparing card game between a player and dealer and played with one or more French decks of 52 cards.

The player is dealt an initial two card hand with the option of drawing cards to bring the total value to 21 or less without exceeding it, so that the dealer will lose by having a lesser hand than the player or by exceeding 21. Many rule variations of blackjack exist. Since the 1960s, blackjack has been a high profile target of advantage players, particularly card counters, who track the profile of cards yet to be dealt, and adapt their wager and playing strategy accordingly.

Other casino games inspired by blackjack include Spanish 21 and pontoon. The recreational British card game of black jack is a shedding-type game and unrelated to the subject of this article.

Rules of play

At a casino blackjack table, the dealer faces between five to seven playing positions from behind a semicircular table. At the beginning of each round, up to three players place their bets in the "betting box" at each position in play. The player whose bet is at the front of the betting box is deemed to have control over the position, and the dealer will consult the controlling player for playing decisions regarding the hand; the other players of that box are said to "play behind". Any player is usually allowed to control or bet in as many boxes as desired at a single table, but it is prohibited to play on more than one table at a time or to place multiple bets in a single box.

Each box is dealt an initial hand of two cards visible to the people playing on it, and often to any other players. The dealer's hand receives its first card face up, and in "hole card" games receives its second card face down immediately (the hole card), which the dealer peeks at but does not reveal unless it makes the dealer's hand a blackjack. Hole card games are sometimes played on tables with a small mirror or electronic sensor which are used to peek securely at the hole card. In European casinos, "no hole card" games are prevalent; the dealer's second card is neither drawn nor consulted until the players have all played their hands.

Cards are dealt either from one or two hand-held decks, from a dealer's shoe. Single cards are dealt to each of wagered-on position clockwise from the dealer's leftmost position, followed by a single card to the dealer, followed by an additional card to each of the positions in play. The players' initial cards may be dealt face-up, or face-down (more common in single-deck games).

The players' object is to win money by creating card totals which will turn out to be higher than the dealer's hand, but without exceeding 21 ("busting"/"breaking"). On their turn, players must choose whether to "hit" (take a card), "stand" (end their turn), "double" (double wager, take a single card and finish), "split" (if the two cards have the same value, separate them to make two hands) or "surrender" (give up a half-bet and retire from the game). Number-cards count as their natural value; the jack, queen, and king (also known as "face cards" or "pictures") count as 10; aces are valued as either 1 or 11 according to the player's best interest. If the hand value exceeds 21 points, it busts, and all bets on it are immediately forfeit. After all boxes have finished playing, the dealer's hand is resolved by drawing cards until the hand busts or achieves a value of 17 or higher (a dealer total of 17 including an ace, or "soft 17", must be drawn to in some games and must stand in others). The dealer never doubles, splits nor surrenders. If the dealer busts, all remaining player hands win. If the dealer does not bust, each remaining bet wins if its hand is higher than the dealer's, and loses if it is lower. In the case of a tied score, known as "push" or "standoff", bets are normally returned without adjustment; however, a blackjack beats any hand which is not a blackjack, even with value 21. Blackjack vs. blackjack is a push. Wins are paid out at 1:1, or equal to the wager, except for winning blackjacks, which are traditionally paid at 3:2, or one and a half times the wager. Many casinos today pay blackjacks at less than 3:2 at some tables.^[3]

Blackjack games almost always provide a side bet called insurance, which may be played when dealer's upcard is an ace. At least one further side bet is usually provided.

Player decisions

After receiving an initial two cards, the player has up to four standard options: "hit," "stand," "double down," or "split". Each option has a corresponding hand signal. Some games give the player a fifth option, "surrender".

• Hit: Take another card from the dealer.

signal: (handheld) Scrape cards against table. (face up) Tap the table or wave hand toward body

• Stand: Take no more cards; also known as "stand pat", "stick", or "stay".

signal: (handheld) Slide cards under chips. (face up) Wave hand horizontally.

• Double down (only available as first decision of a hand): The player is allowed to increase the initial bet by up to 100% in exchange for committing to stand after receiving exactly one more card. The additional bet is placed in the betting box next to the original bet. Some games do not permit the player to increase the bet by amounts other than 100%. Non-controlling players may double their wager or decline to do so, but they are bound by the controlling player's decision to take only one card.

signal: Place additional chips beside the original bet, and point with one finger.

• Split (only available as first decision of a hand): If the first two cards have the same value, the player can split them into two hands, by moving a second bet equal to the first into an area outside the betting box of the original bet. The dealer separates the two cards and draws a further card on each, placing one bet with each hand. The player then plays out the two separate hands in turn, with some restrictions. Occasionally, in the case of ten-valued cards, some casinos allow splitting only when the cards have the identical ranks; for instance, a hand of T-T may be split, but not of T-K. However, usually all ten-value cards are treated the same. Doubling and further splitting of post-split hands may be restricted, and blackjacks after a split are counted as non-blackjack 21 when comparing against the dealer's hand. Hitting split aces is usually not allowed. Non-controlling players may follow the controlling player by putting down an additional bet, or decline to do so, instead associating their existing wager with one of the two post-split hands. In that case they must choose which hand to play behind before the second cards are drawn.

signal: Place additional chips next to the original bet outside the betting box. Point with two fingers spread into a V formation.

• Surrender (only available as first decision of a hand): Some games offer the option to "surrender", usually in hole card games and directly after the dealer has checked for blackjack (but see below for variations). When the player surrenders, the house takes half the player's bet and return the other half to the player; this terminates the player's interest in the hand. The request to surrender is made verbally, there being no standard hand signal.

Each hand may normally "hit" as many times as desired so long as the total is not above hard-20. On reaching 21 (including soft 21), the hand is normally required to stand; busting is an irrevocable loss and the players' wagers are immediately forfeited to the house. After a bust or a stand, play proceeds to the next hand clockwise around the table. When the last hand has finished being played, the dealer reveals the hole card, and stands or draws further cards according to the rules of the game for dealer drawing. When the outcome of the dealer's hand is established, any hands with bets remaining on the table are resolved (usually in counter-clockwise order): bets on losing hands are forfeited, the bet on a push is left on the table, and winners are paid out.

Insurance

If the dealer's upcard is an ace, the player is offered the option of taking "insurance" before the dealer checks the hole card.

Insurance is a side bet that the dealer has blackjack and is treated independently of the main wager. It pays 2:1 and is available when the dealer's exposed card is an ace. The idea is that the dealer's second card has a fairly high probability (nearly one-third) to be ten-valued, giving the dealer blackjack and disappointment for the player. It is attractive (although not necessarily wise) for the player to insure against the possibility of a dealer blackjack by making a maximum "insurance" bet, in which case the "insurance proceeds" will make up for the concomitant loss on the original bet. The player may add up to half the value of their original bet to the insurance and these extra chips are placed on a portion of the table usually marked "Insurance Pays 2 to 1".

Players with a blackjack may also take insurance, and in taking maximum insurance they commit themselves to winning an amount exactly equal to their main wager, regardless of the dealer's outcome. Fully insuring a blackjack against blackjack is thus referred to as "taking even money", and paid out immediately, before the dealer's hand is resolved; the players need not produce to place more chips for the insurance wager.

Insurance bets are expected to lose money in the long run, because the dealer is likely to have blackjack less than one-third of the time. However the insurance outcome is strongly anti-correlated with that of the main wager, and if the player's priority is to reduce variation, it is reasonable to pay for this.

Furthermore, the insurance bet is susceptible to advantage play. It is advantageous to make an insurance bet whenever the hole card has more than a chance of one in three of being a ten. Advantage play techniques can sometimes identify such situations. In a multi-hand, face-up, single deck game, it is possible to establish whether insurance is a good bet simply by observing the other cards on the table after the deal; even if there are just 2 player hands exposed, and neither of their two initial cards is a ten, then 16 in 47 of the remaining cards are tens, which is larger than 1 in 3, so insurance is a good bet. This is an elementary example of the family of advantage play techniques known as card counting.

Bets to insure against blackjack are slightly less likely to be advantageous than insurance bets in general, since the ten in the player's blackjack makes it less likely that the dealer has blackjack too.