Blackjack, also known as twenty-one, is the most widely played casino banking game in the world. Blackjack is a comparing card game between a player and dealer, meaning that players compete against the dealer but not against any other players. It is played with one or more decks of 52 cards. The object of the game is to beat the dealer, which can be done in a number of ways:

• Get 21 points on the player’s first two cards (called a blackjack), without a dealer blackjack;
• Reach a final score higher than the dealer without exceeding 21; or
• Let the dealer draw additional cards until his or her hand exceeds 21.

The player or players are dealt an initial two-card hand and add together the value of their cards. Face cards (kings, queens, and jacks) are counted as ten points. A player and the dealer can count his or her own ace as 1 point or 11 points. All other cards are counted as the numeric value shown on the card. After receiving their initial two cards, players have the option of getting a “hit”, or taking an additional card. In a given round, the player or the dealer wins by having a score of 21 or by having the highest score that is less than 21. Scoring higher than 21 (called “busting” or “going bust”) results in a loss. A player may win by having any final score equal to or less than 21 if the dealer busts. If a player holds an ace valued as 11, the hand is called “soft”, meaning that the player cannot go bust by taking an additional card; 11 plus the value of any other card will always be less than or equal to 21. Otherwise, the hand is “hard”.

The dealer has to take hits until his or her cards total 17 or more points. (In some casinos the dealer also hits on a “soft” 17, e.g. an initial ace and six.) Players win if they do not bust and have a total that is higher than the dealer’s. The dealer loses if he or she busts or has a lesser hand than the player who has not busted. If the player and dealer have the same total, this is called a “push” and the player typically does not win or lose money on that hand.

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 that have been dealt and adapt their wagers and playing strategies 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.

History

Blackjack’s precursor was twenty-one, a game of unknown origin. The first written reference is found in a book by the Spanish author Miguel de Cervantes, most famous for writing Don Quixote. Cervantes was a gambler, and the main characters of his tale Rinconete y Cortadillo, from Novelas Ejemplares, are a couple of cheats working in Seville. They are proficient at cheating at ventiuna (Spanish for twenty-one), and state that the object of the game is to reach 21 points without going over and that the ace values 1 or 11. The game is played with the Spanish baraja deck, which lacks eights, nines and tens. This short story was written between 1601 and 1602, implying that ventiuna was played in Castilla since the beginning of the 17th century or earlier. Later references to this game are found in France and Spain.

When twenty-one was introduced in the United States, gambling houses offered bonus payouts to stimulate players’ interest. One such bonus was a ten-to-one 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 to the game, even though the ten-to-one bonus was soon withdrawn. In the modern game, a blackjack refers to any hand of an ace plus a ten or face card, regardless of suits or colours.

Rules of play at casinos

At a casino blackjack table, the dealer faces five to seven playing positions from behind a semicircular table. Between one and eight standard 52-card decks are shuffled together. At the beginning of each round, up to three players can place their bets in the “betting box” at each position in play. That is, there could be up to three players at each position at a table in jurisdictions that allow back betting. 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 for an individual to play on more than one table at a time or to place multiple bets within a single box. In many U.S. casinos, however, players are limited to playing two or three positions at a table and often only one person is allowed to bet on each position.

The dealer deals cards from his/her left (the position on the dealer’s far left is often referred to as “first base”) to his/her far right (“third base”). 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 immediately receives its second card face down (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 that is 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 handheld decks, from a dealer’s shoe, or from a shuffling machine. Single cards are dealt to each wagered-on position clockwise from the dealer’s left, 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 that turn out to be higher than the dealer’s hand but do not exceed 21 (“busting”/”breaking”), or alternatively by allowing the dealer to take additional cards until he/she busts. 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 choice. 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, or 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 that is not a blackjack, even one with a value of 21. An outcome of blackjack vs. blackjack results in 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 (meaning the player receives three dollars for every two bet), or one-and-a-half times the wager. Many casinos today pay blackjacks at less than 3:2 at some tables.

Blackjack games almost always provide a side bet called insurance, which may be played when dealer’s upcard is an ace. Additional side bets, such as “Dealer Match” which pays when the player’s cards match the dealer’s up card, are sometimes available.

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: Scrape cards against table (in handheld games); tap the table with finger or wave hand toward body (in games dealt face up).

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

Signal: Slide cards under chips (in handheld games); wave hand horizontally (in games dealt face up).

• Double down: 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 outside the betting box, and point with one finger.

• Split: If the first two cards of a hand 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. The dealer separates the two cards and draws an additional card on each, placing one bet with each hand. The player then plays out the two separate hands in turn; except for a few restrictions, the hands are treated as independent new hands, with the player winning or losing their wager separately for each hand. 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 10-10 may be split, but not one of 10-king. However, usually all 10-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. Some casinos do not give non-controlling players this option, and require that the wager of a player not electing to split remains with the first of the two post-split hands.

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 returns 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.

Hand signals are used to assist the “eye in the sky“, a person or video camera located above the table and sometimes concealed behind one-way glass. The eye in the sky usually makes a video recording of the table, which helps in resolving disputes and identifying dealer mistakes, and is also used to protect the casino against dealers who steal chips or players who cheat. The recording can further be used to identify advantage players whose activities, while legal, make them undesirable customers. In the event of a disagreement between a player’s hand signals and their words, the hand signal takes precedence.

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 counterclockwise 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 (meaning that the player receives two dollars for every dollar bet) 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 do not need 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.

Rule variations and their consequences for the house edge

The rules of casino blackjack are generally determined by law or regulation, which establishes certain rule variations allowed at the discretion of the casino. The rules of any particular game are generally posted on or near the table, failing which there is an expectation that casino staff will provide them on request. Over 100 variations of blackjack have been documented.

As with all casino games, blackjack incorporates a “house edge”, a statistical advantage for the casino that is built into the game. The advantage of the dealer’s position in blackjack relative to the player comes from the fact that if the player busts, the player loses, regardless of whether the dealer subsequently busts. Nonetheless, blackjack players using basic strategy will lose less than 1% of their total wagered amount with strictly average luck; this is very favorable to the player compared to other casino games. The loss rate of players who deviate from basic strategy through ignorance is generally expected to be greater.

• Dealer hits soft 17

Each game has a rule about whether the dealer must hit or stand on soft 17, which is generally printed on the table surface. The variation where the dealer must hit soft 17 is abbreviated “H17” in blackjack literature, with “S17” used for the stand-on-soft-17 variation. Substituting an “H17” rule with an “S17” rule in a game benefits the player, decreasing the house edge by about 0.2%.

• Number of decks

All things being equal, using fewer decks decreases the house edge. This mainly reflects an increased likelihood of player blackjack, since if the players draws a ten on their first card, the subsequent probability of drawing an ace is higher with fewer decks. It also reflects a decreased likelihood of blackjack-blackjack push in a game with fewer decks.

Casinos generally compensate by tightening other rules in games with fewer decks, in order to preserve the house edge or discourage play altogether. When offering single deck blackjack games, casinos are more likely to disallow doubling on soft hands or after splitting, to restrict resplitting, require higher minimum bets, and to pay the player less than 3:2 for a winning blackjack.

The following table illustrates the mathematical effect on the house edge of the number of decks, by considering games with various deck counts under the following ruleset: double after split allowed, resplit to four hands allowed, no hitting split aces, no surrender, double on any two cards, original bets only lost on dealer blackjack, dealer hits soft 17, and cut-card used. The increase in house edge per unit increase in the number of decks is most dramatic when comparing the single deck game to the two-deck game, and becomes progressively smaller as more decks are added.

Number of Decks	House Advantage
Single deck	0.17%
Double deck	0.46%
Four decks	0.60%
Six decks	0.64%
Eight decks	0.65%

Late/early surrender

Surrender, for those games that allow it, is usually not permitted against a dealer blackjack; if the dealer’s first card is an ace or ten, the hole card is checked to make sure there is no blackjack before surrender is offered. This rule protocol is consequently known as “late” surrender. The alternative, “early” surrender, gives player the option to surrender before the dealer checks for blackjack, or in a no-hole-card game. Early surrender is much more favorable to the player than late surrender. Most medium-strength hands should be surrendered against a dealer Ace if the hole card has not been checked.

For late surrender, however, while it is tempting opt for surrender on any hand which will probably lose, the correct strategy is to only surrender on the very worst hands, because having even a one in four chance of winning the full bet is better than losing half the bet and pushing the other half, as entailed by surrendering.

Resplitting

If the cards of a post-split hand have the same value, most games allow the player to split again, or “resplit”. The player places a further wager and the dealer separates the new pair dealing a further card to each as before. Some games allow unlimited resplitting, while others may limit it to a certain number of hands, such as four hands (for example, “resplit to 4”).

Hit/resplit split aces

After splitting aces, the common rule is that only one card will be dealt to each ace; the player cannot split, double, or take another hit on either hand. Rule variants include allowing resplitting aces or allowing the player to hit split aces. Games allowing aces to be resplit are not uncommon, but those allowing the player to hit split aces are extremely rare. Allowing the player to hit hands resulting from split aces reduces the house edge by about 0.13%; allowing resplitting of aces reduces house edge by about 0.03%. Note that a ten-value card dealt on a split ace (or vice versa) is a “soft 21” and not a “natural”.

No double after split

After a split, most games allow doubling down on the new two-card hands. Disallowing doubling after a split increases the house edge by about 0.12%.

Double on 9/10/11 or 10/11 only

Under the “Reno rule”, double down is only permitted on hard totals of 9, 10, or 11 (under a similar European rule, only 10 or 11). Basic strategy would otherwise call for some doubling down with hard 9 and soft 13–18, and advanced players can identify situations where doubling on soft 19–20 and hard 8,7 and even 6 is advantageous. The Reno rule prevents the player from taking advantage of double down in these situations and thereby increases the player’s expected loss. The Reno rule increases the house edge by around one in 1000, and its European version by around two in 1000.

No hole card and OBO

In most non-U.S. casinos, a ‘no hole card’ game is played, meaning that the dealer does not draw nor consult his or her second card until after all players have finished making decisions. With no hole card, it is almost never correct basic strategy to double or split against a dealer ten or ace, since a dealer blackjack will result in the loss of the split and double bets; the only exception is with a pair of A’s against a dealer 10, where it is still correct to split. In all other cases, a stand, hit or surrender is called for. For instance, holding 11 against a dealer 10, the correct strategy is to double in a hole card game (where the player knows the dealer’s second card is not an ace), but to hit in a no hole card game. The no hole card rule adds approximately 0.11% to the house edge.

The “original bets only” rule variation appearing in certain no hole card games states that if the player’s hand loses to a dealer blackjack, only the mandatory initial bet (“original”) is forfeited, and all optional bets, meaning doubles and splits, are pushed. “Original bets only” is also known by the acronym OBO; it has the same effect on basic strategy and house edge as reverting to a hole card game.

Altered payout for a winning blackjack

In many casinos, a blackjack pays only 6:5 or even 1:1 instead of the usual 3:2. This is usually at tables with the lowest table minimums and single-deck games. Among common rule variations in the U.S., these altered payouts for blackjack are the most damaging to the player, causing the greatest increase in house edge. Since blackjack occurs in approximately 4.8% of hands, the 1:1 game increases the house edge by 2.3%, while the 6:5 game adds 1.4% to the house edge. Video blackjack machines generally pay 1:1 payout for a blackjack. The 6:5 rule is most commonly employed on table blackjack at single deck games, where they help the house to compensate for low house edge intrinsic in using one deck only.

Dealer wins ties

The rule that bets on tied hands are lost rather than pushed is catastrophic to the player. Though rarely used in standard blackjack, it is sometimes seen in “blackjack-like” games such as in some charity casinos.