Rules of Poker
- Players, Cards and Objective
- Ranking of Poker Hands
- The Deal and the Betting
- The Showdown
- Poker Variants
- Other Web Sites Poker Rules and Glossaries
This aim of this page is to give a general idea of how poker is played. For those who need greater detail on particular aspects of the rules there are more detailed pages on hand ranking, the betting process, and on the specifics of particular types of poker such as Seven Card Stud, Texas Hold'em, Omaha and Draw Poker, which are listed on the Poker Variants page.
Traditionally, poker has been thought of as a game for 2 to 7 players, the more the better with 6 or 7 being the ideal number. However, some variants can be played by more than seven, and some versions work well for a small number of players - even with just two ("heads up"). The deal and play are clockwise.
A standard international 52-card pack is used, and in most forms of poker there are no jokers. The rank of the cards, from high to low, is A, K, Q, J, 10, 9, 8, 7, 6, 5, 4, 3, 2. In certain circumstances the ace can be used as a low card, below the 2. For the purposes of comparing hands all suits are equal.
Poker is normally played for money, but it is convenient to use chips to represent money during the actual games. These generally come in various denominations, sometimes labelled with numbers 1, 2, 5, 10, 20, 50, 100, 500, and sometimes in colours such as white, red, blue whose values must be agreed. Players buy chips from the host before the game and redeem them for money at the end.
In outline, a game of poker is played as follows.
- Players place an initial bet, if required by the game being played, by placing some of their chips into a central common area known as the pot or pool.
- The dealer deals cards to the players.
- Players look at their own cards, and have the opportunity to increase their bet, placing extra chips into the pot. All other players must then either put in chips to bring their contribution to the pot to at least an equal amount or fold, discarding their cards and abandoning any chips they have so far contributed to the pot.
- Players who have not folded are known as active players. When all active players have contributed an equal amount to the pot, that is the end of the betting round. According to the variant being played, further cards may be dealt or players may have an opportunity to exchange some cards, after which there is another betting round, and so on.
- When the last betting round has ended there is a showdown: all active players show their cards, and the owner of the best five-card hand takes the contents of the pot. If at any point only one active player remains, that player automatically wins the pot without having to show any cards.
The objective is of course to win money, and there are two ways to do this.
- To have the best five-card hand at the showdown.
- To persuade all the other players to fold.
It is the second possibility that creates the possibility for bluff. If everyone believes that you probably have a good hand, then when you raise the stake, they may prefer to fold rather than add chips to a pot that they will probably lose. If they all fold you win, even though your hand may in reality be much worse than theirs.
It is of course necessary to know which hands beat which. A poker hand always consists of five cards. Even though in some variants you may have more than five cards to choose from, you select the five cards that make the best hand, and for the purpose of comparing hands any other cards are irrelevant.
The ranking of hands from high to low in standard poker is as follows.
- Straight Flush
- Five consecutive cards of the same suit. Ace can be counted as high to make the highest type of Straight Flush, which is A-K-Q-J-10 of a suit, sometimes known as a Royal Flush. Ace can be low to make (5-4-3-2-A), but not high and low at the same time (for example 2-A-K-Q-J is not valid).
- Four of a Kind
- Four cards of the same rank and one other card, such as 9-9-9-9-Q. Four of a kind are sometimes known as quads or in some non-English speaking countries as a poker. The odd card - the queen in the example - is called the kicker.
- Full House
- Three cards of the same rank plus a pair of cards of another rank, such as 5-5-5-K-K, which would be described as "fives full of kings". A full house is sometimes known as a boat.
- A flush consists of five cards of the same suit (not all consecutive, otherwise it would be a straight flush).
- Five cards of consecutive ranks, not all of the same suit. The highest is A-K-Q-J-10 and an Ace can instead be counted as low to make the lowest straight 5-4-3-2-A, which is sometimes called a wheel. An Ace cannot be in the interior of a straight - for example 4-3-2-A-K is not a straight.
- Three of a Kind
- Three cards of the same rank and two cards of different ranks - for example 7-7-7-10-6. This is sometimes known as a triplet or trips.
- Two Pair
- Two cards of one rank, two cards of a second rank and one card of a third rank (the kicker) - for example J-J-3-3-8.
- Two cards of equal rank and three cards of different ranks - for example Q-Q-A-8-7.
- A hand which does not fit any of the categories above, commonly known as High Card or sometimes No Pair. That is: five cards of different ranks, not all consecutive and not all the same suit.
Any hand of a higher type beats any hand of a lower type. When comparing two hands of the same type, the ranking is determined by the ranks of the individual cards. The most numerous rank of cards in each hand (the quad, the triplet in a full house or trips, otherwise the pair if any) is compared first; if these are equal, any less numerous ranks are compared. When two ranks are equally numerous, the highest-ranking cards are compared before the others.
Note that in standard poker the four suits are all equal, and that poker hands consist of five cards only. Therefore if two players can make five-card hands that are equal apart from the suits of the cards, there is a tie and if necessary they share the winnings equally.
- 4-4-4-7-7 beats 2-2-2-K-K because 4 is higher than 2, and 4-4-4-3-2 beats 2-2-2-A-K for the same reason.
- 5-5-5-9-9 beats 5-5-5-6-6 (these two hands could appear together in a game with shared cards or wild cards).
- K-K-2-2-A beats 7-7-3-3-Q, because the higher-ranking pairs are compared first and kings beat 7's.
- 7-7-3-3-Q beats 7-7-2-2-A because the higher-ranking pairs are equal, and the 3's beat the 2's.
- A-K-8-3-2 beats A-K-7-6-5, because 8 is higher than 7, the highest two cards in each hand being equal.
- 6-5-4-3-2 beats 5-4-3-2-A, because the ace must be low to make the straight.
The hand ranking above applies to standard poker. There are modifications to this in certain types of poker variant, for example:
- games with wild cards - cards that can be used to represent a card of any suit or rank;
- low poker or lowball games, in which the lowest ranking hand wins - also in the low component of high-low games in which the highest and lowest hands share the pot;
- games with stripped decks - decks of less than 52 cards obtained by removing the lowest card.
The poker hand ranking page gives further details.
- Before starting a game of poker, it is necessary to decide what variation of poker will be played, and for what stakes.
- If you play in a formal game in a casino or tournament or play online, then these decisions have already been made by the host and by joining a table you accept the game and stakes played at that table.
- The same applies if you join an established private game: you need to find out from the host what rules and stakes are used, and abide by them.
- If you play regularly with the same group of people, you have probably settled these questions in the past and if you always play the same way, no discussion may be needed.
- However, if any new players join your group it is important to make sure at the outset that everyone understands the house rules and stakes. Different players may be used to different arrangements, and disputes that arise during a game from genuine rule misunderstandings can be very difficult to resolve fairly and amicably.
- Which variation?
- There are hundreds of different varieties of poker - see the variants section below for a summary of some popular ones, and the poker variants page for a fuller list.
- Casino card room games and online games are normally devoted to a single type of poker. Some private games are like this too, but many prefer to play "dealer's choice", in which each player may choose a different variant at his or her turn to deal.
- What stakes?
- Poker is to a large extent a game of money management, so the betting structure has a significant effect on the tactics of the game. This question breaks down into several parts.
- How much must you pay to the pot in order to be dealt a hand? This may typically be in the form of an ante, paid by every player before each deal, or a blind, paid by only one or two players, the turn to pay rotating with the deal.
- What is the minimum and maximum amount of each bet (or raise)? At one extreme these could be fixed in size, and at the other there could be no upper limit, with various possibilities in between.
- How many times may a bet be raised within a betting round? In some groups there is a limit of three or four raises; in others the players can go on raising each other indefinitely.
- For details of possible betting structures, see the poker betting page.
- Whatever betting limits are agreed, and whether you are using poker chips as recommended or playing directly for cash, poker is nowadays usually played for table stakes. This means that a player cannot introduce extra money into the game during a hand. Once the deal has begun, you can only bet using the chips (or money) you had in front of you, clearly displayed on the table, at the start of the deal.
- A minimum buy-in should be agreed - this might typically be from 10 to 20 times the minimum bet. A player joining the game should begin with at least this value of chips on the table. After a pot has been won and before the next deal, a player may add more chips, but chips cannot be taken out of the game unless the player leaves the game altogether.
When poker is played in a casino, the house makes a charge for providing the table, cards and dealer. This is paid in chips and may take the form either of a time collection from each player, for example every half hour in advance, or a rake, a percentage of each pot retained by the house. Online poker rooms also normally take a rake from each pot.
Since in most poker games the dealer has a positional advantage, the first dealer is chosen at random. Traditionally, one of the players deals cards face up one at a time from a shuffled deck and the dealer is the first player who receives a Jack. Since this method slightly favours those who receive their cards first, players may prefer to deal just one card each and the highest deals. If two players receive equal highest cards, suits rank in the order spades (high), hearts, diamonds, clubs (low) (but note that this suit order is not used to break ties between hands in the showdown).
Before each deal, some or all players must place an initial stake in the pot as agreed. The simplest arrangement is that each player puts in an equal amount, known as the ante.
The dealer then shuffles the cards thoroughly and offers them to the player to the right to cut. If this player declines to cut, any other player may cut. When the cards are cut, each portion of the pack must contain at least five cards.
Note: The position of the dealer is often marked by a token called the dealer button which is passed to the left after each hand. In a formal game, for example in a casino or tournament, the house will normally provide a professional dealer who does not play, but shuffles and deals every hand on behalf of the player with the dealer button. In this case, often there is no cut. The dealer also looks after the pot and the discards, and generally makes sure that the game proceeds smoothly and the rules are observed. When poker is played on line, the virtual cards are of course shuffled and dealt by the server computer. In what follows, "dealer" means the player who currently has the dealer button, irrespective of who actually deals the cards.
The cards are dealt as required by the rules of the particular variant being played. In formal games, each stage of the deal is normally begun by burning a card - that is, dealing the top card of the pack face down - before dealing cards to the players or the table. In casinos the dealer slides the burned cards under the pile of chips that constitutes the pot.
At various points during or after the deal there will be a betting round. The details of when these betting rounds occur depend on the variant being played, but the principles are always the same. During the betting round all dealing, exchange of cards, etc. is suspended, and the players have an opportunity to increase their bets.
In most variants the first betting round is begun by the player to the left of the dealer if all the players have placed an equal ante in the pot. If only some of the players have put chips in the pot - for example in a game played with blinds - then the round is begun by the player to the left of the player(s) who have already put in a stake. The second and subsequent betting rounds may, according to the variant, be begun by the nearest active player to the left of the dealer seat, or by a player determined by the action in the previous betting round. In variants where some cards are dealt face up, each betting round may begin with the player who has the best (or worst) hand showing.
The players act in clockwise order around the table, continuing for as many circuits as are necessary, missing any players who have dropped out, until all active players have had a turn and the stakes of all the active players are equal.
If no one has bet so far in the current betting round, and the value of chips contributed by all active players is equal, you have two options at your turn:
- You do not wish to bet more chips at the moment, but you remain active and reserve the right to take part in future betting. In the first betting round, when all players have contributed an equal ante, players often say pass rather than check.
- You bet some more chips by pushing them towards the pot. The amount must be between the minimum and maximum limits currently in force. The player who bets first in the first betting round is said to open the betting.
If you have fewer chips in the pot than some other player, either because there has been a bet in the current betting round, or in the first round when some of the players placed blinds, you have three options:
- You drop out of the hand, discarding your cards face down onto a discard pile, which is known as the muck. No player is allowed to see the cards you discarded. You will take no further part until the next deal, and any chips you have placed in the pot are lost to the eventual winner of the pot.
- In order to remain active, you match the latest bet or raise, but you do not try to increase it further. You push towards the pot enough chips so that your total contribution to the pot equals that of the player who last bet or raised.
- You increase the bet by pushing towards the pot the amount that you would need to call plus the value of your raise. The value of your raise must be between the minimum and maximum limits.
The betting round ends when either all the active players check, or all the other active players call the last bet or raise, or there is only one active player remaining.
- If only one active player remains, this player immediately wins the pot, and does not have to show his or her cards (except in a few variants that have a minimum hand requirement to win the pot). The deal is over, the cards are collected, and if the session is to continue the players ante for a new deal.
- If more than one active player remains the game continues to the next stage - a continuation of the deal, an opportunity to exchange cards, or a showdown, according to the variant.
Example. Six players: A, B, C, D, E, F. All place $1 ante. In the first betting round A checks, B bets $2 and C folds. Now D raises $4. In order to do this D has to contribute $6 worth of chips: $2 to match B's bet and another $4 for the raise. E calls, which costs $6, the amount needed to match what D has put in. Suppose F wants to raise another $4. F must produce $10 in chips: $6 to match what D has put in plus $4 for the raise. It is now A's turn and it would cost A $10 to call: A decides to fold. B calls, which costs $8, the difference between the $2 B already put in and F's $10. C is already out so does not get a turn. Having already put in $6, D could call for $4, but decides to fold. E calls for $4, the difference between F's $10 and the $6 that E has already put in. That ends the betting round, because the three active players B, E and F have each put in $10 in this round. F, who was the last to raise, does not get another turn. These three $10 bets plus the $6 from D are combined with the antes to form a pot of $42.
In practice, most betting rounds are much less eventful than this. Not infrequently one player will bet, all the others will fold, and that player will collect the pot, winning no more than the other players' antes.
It is important that at their turn players clearly state what they are doing, by saying "call", "raise", etc. or by making an unambiguous gesture of pushing chips towards the pot or discarding their cards. Having indicated what you are going to do you are not allowed to change your mind. In particular you must not make what is known as a "string raise": match the previous bet as though calling, pause to observe the reactions of the other players, and then add a raise.
When playing with table stakes it sometimes happens that a player wishing to call has insufficient chips to match the latest bet or raise. In such a case the player can call by putting in all his or her remaining chips. The player is then "all-in", and is entitled to take part in the showdown without contributing any further chips, but the amount that can be won from each opponent is limited to the value of chips that the "all in" player has contributed to the pot. To achieve this, the pot is split into two. The main pot consists of the chips contributed by each player, up to the amount put in by the "all-in" player. All excess chips form a side pot, from which the "all-in" player is excluded. If there is more than one active player who is not "all-in", they can continue placing bets in this side pot. If other players also go "all-in", further side pots will be created in the same way.
Further details of side pots, betting limits, blind bets and other details can be found on the poker betting page.
In theory this is simple.
- If all players except one have folded there is no showdown. The single surviving player simply takes the pot without having to show any cards.
- If there is more than one active player at the end, they all show their cards to everyone, and the holder of the best hand (according to the hand ranking explained above) wins the whole pot.
- If two or more active players turn out to have equally good hands, beating all the others, they share the contents of the pot equally between them.
If there are side pots, because some players were "all in", these are settled in reverse order, beginning with the one that was created most recently.
In practice some complications can arise. For a more detailed discussion, including the treatment of split pots, see the showdown section of the poker betting page. This page also deals procedures for declaring which part of the pot one is playing for in high-low and other split pot variants.
Some players are reluctant to be the first to show their cards: they would prefer to wait to see the other players' hands and then show their own cards only if they can win. To avoid a stalemate between such players, the rule is that the player who was the last to take positive action (bet or raise) in the final betting round must show first, followed by the other active players in clockwise order. If everyone checked in the last betting round, the first active player to the left of the dealer seat shows first. Despite this rule, to speed up the game, active players are encouraged to show their cards immediately rather than waiting for their turn.
In a showdown, players showing a hand must expose the whole of their hand. It is not sufficient to show just enough cards to prove that one has a good enough hand to win, and not just the five cards that are being used to make one's best hand. In the showdown, players must show all the cards they were dealt, all at once, so that everyone at the table can see what they have.
Players who expose their hands quite often also announce what type of hand they have, but in some cases a player may overlook some combination and announce a weaker hand than he or she really has. This would be unlikely to happen in a straightforward game with five-card hands, but in variants where players select the best hand from seven or more cards, or where wild cards are involved, it is not uncommon for some better possibility to be missed. In formal poker games the usual rule is that "the cards speak for themselves". This means that when a player's hand is exposed at the showdown, it counts as the best five-card hand that can be made from it, even if the owner of the hand does not find it. It is the duty of the dealer or any other player who notices to point out what the best hand is, and it is treated as such, irrespective of how the owner described it. In some private games, however, the reverse rule is used: that players must declare what their hand is, and provided that the hand they declare can be made from the cards shown, that is how it is treated even if a better hand was available.
Some players prefer to muck (discard) their cards without showing them when they can see that they are beaten. This is normal practice, but the traditional rule is that any player who was dealt a hand, even a player who has folded, has the right to see the hand of any player who was involved in the showdown. The purpose of the rule is primarily to enable collusion between players to be exposed, and it is considered poor etiquette to insist on the right to see a discarded hand without good reason. In particular a player should not continually demand to see another player's hand so as to analyse that player's style of play or simply to irritate the player, and the winner of the pot should not ask to see a loser's hand. Formal poker games often have the rule that the right to see discarded hands at showdown can be revoked if overused by a player.
Here is a quick summary of some of the best known poker variants. A fuller list will be found on the Poker Variants page.
Five Card Draw is one of the oldest and best known poker games, but has been superseded in popularity by some of the newer styles. Each player is dealt a private hand of five cards. Players look at their cards and there is a first betting round, begun by the player to the left of dealer's seat. If all pass (check), the cards are thrown in, the dealer button is passed to the left and a new ante is added to the pot.
If the betting was opened, then after the first betting round each player in turn can discard any number of cards face down, and is dealt an equal number of replacement cards. Then there is a second betting round, begun by the player who opened the betting in the first round, or if this player has folded by the nearest active player to the opener's left. If more than one player survives the second round, there is a showdown.
Five Card Draw is often played with a minimum requirement of a pair of jacks to open ("Jacks or Better"). Sometimes it is played with a 53-card pack containing a joker, which is used as a wild card (which can be used to represent any card) or a bug (which can represent an ace or complete a straight or flush). For further details and variants, see the Draw Poker page.
In Lowball or Low Poker, the lowest ranking hand wins the pot. Players need to agree whether aces can be counted as low for this purpose, and whether straights and flushes count. Depending on the answers, the best possible hand will be 5-4-3-2-A or 6-4-3-2-A (mixed suits) or 7-5-4-3-2 (mixed suits). Note that in a "High Card" hand the cards are still compared in order from highest to lowest, so in Lowball 8-6-5-4-3 beats 8-7-4-3-2 because 6 is lower than 7. The mechanism - deal, draw and betting rounds - are essentially the same as in Draw Poker.
Low versions of other poker variants can also be played, and it is also possible to play that there are two winners, the holders of the highest and lowest hands splitting the pot.
In stud poker games, some of the cards are dealt face up, and there are several betting rounds during the deal.
In Five Card Stud, the dealer begins by dealing one card face down to each player (the hole card) and then one card face up. Players may look at their own hole cards. The first betting round is begun by the player with the highest face-up card. Five Card Stud is sometimes played without an ante, in which case the player with the highest card showing must open with a minimum bet. When the betting round is complete the dealer deals another face up card to each player and there is another betting round, begun by the player who currently has the best hand showing. This is repeated until each player has five cards - one face down and four face up - and after the final betting round there is a showdown between the survivors.
Seven Card Stud is nowadays more popular than five-card. Each player is dealt (one card at a time) two face down hole cards and one card face up. The first betting round was traditionally begun by the player with the highest card showing, but some groups play that the player showing the lowest card must open with a compulsory bet, called the bring-in. After the first betting round one face up card (fourth street) is dealt to each player and there is a second betting round, this time always started by the highest hand showing. This procedure is repeated for the next two face up cards (fifth and sixth street). The final card (seventh street) is dealt face down, so that each player has four cards showing and three private cards - the first, second and last. After a final round of betting there is a showdown in which the active players show all their cards, and the winner is the player whose hand includes five cards that make the best poker hand.
Razz is a lowball version of Seven Card Stud, in which the lowest five-card poker hand wins the pot. Aces can be used as low cards and flushes and straights do not count, so the lowest hand is 5-4-3-2-A. The first betting round is begun with a compulsory bet by owner of the highest card showing; subsequent rounds are begun by the owner of the lowest hand showing.
Seven Card Stud is often played high-low. The procedure is the same as in ordinary Seven Card Stud except that at the showdown the pot is shared equally between the highest and lowest five-card hands. A player can use a different subset of five cards to compete for high and low, thus winning both parts of the pot. It's also possible to win the whole pot with the same five cards - for example if those cards form a straight or flush, which counts for high but not for low. Often this game is played with a rule that a five-card hand must have no card higher than 8 to qualify to win the low half of the pot - this version is known as Eight or Better. If no one qualifies for low, the high hand takes the whole pot.
Shared Card Poker
In Shared Card or Community Card games, some cards are dealt face up to the centre of the table and can be used by all players as a part of their hand. The best known game of this type is Texas Hold'em, which thanks to televised tournaments towards at the end of the 20th century and its success as an online game at the start of the 21st has become one of the most popular poker variants.
In Texas Hold'em the pot is normally started with blinds - forced bets of a given size placed by the players to the left of the dealer seat. Typically the player to the immediate left of the dealer seat must place a small blind and the next player to the left must place a big blind of twice this amount. Each player is then dealt two cards face down and there is a betting round begun by the player to the left of the big blind. The blind bets are treated like ordinary bets, in that players must match the big blind in order to call and can raise by putting in a greater amount. When it comes to the big blind player's first turn, this player is allowed to raise even if the active players' bets are currently equal, the others having done no more than call.
After the first betting round the dealer deals three cards face up to the table, after which there is a second betting round. These three face up cards are known as the flop. The dealer then deals a fourth face up card, the turn, and there is a third betting round, and then a fifth face up card, the river, followed by a fourth and final betting round. All betting rounds other than the first are begun by the first active player to the left of the dealer seat. In the showdown each player has seven cards available from which to make the best five-card poker hand: two hole cards and the five face-up cards which are available to everyone. This can quite often result in split pots. For example the cards on the table are 5-5-5-K-7, one player has A-K and another has K-3. Since only five cards can be used, in the showdown these players have full houses 5-5-5-K-K and they share the pot equally.
Omaha is another well-known shared card game. After the blinds have been placed, each player is dealt four hole cards. The betting and the remainder of the deal is similar to Texas Hold'em: a betting round;, a three-card flop dealt face up; a second betting round; a single "turn" card dealt face up; a third betting round; a single "river" card dealt face up; a final betting round. In the showdown, each player must use exactly two hole cards plus exactly three of the five cards on the table to make the best five-card poker hand.
A popular variant is Omaha Hi-Lo/8, which is played like Omaha except that the pot is shared equally between the highest and lowest hands at the showdown. Players can use different cards for the high and the low, but always two cards from hand and three from the table in each case. When comparing low hands aces are low and straights and flushes do not count. To qualify to win the low part of the pot, none of the five cards can be higher than 8. If there is no qualifying hand the high hand wins the whole pot.
Compendium Poker Games
Rather than sticking continuously to a single poker variant, many players prefer to play several different variants within a single session. For this reason, home poker games are often played as Dealer's Choice. Each dealer in turn announces, before the antes are placed, what variant will be played for that deal only. This way everyone gets a chance to play their favourite version from time to time. Most groups will have a repertoire of variants that they regularly play, so the announcement can be quite brief. Often gambling games that are not stictly types of poker such as Guts, 7-27 or Bourré will be allowed as options.
Casinos and online card rooms also sometimes offer games in which several different poker games are played in succession. A popular example is H.O.R.S.E. in which the five variants Texas Hold'em, Omaha Hi-Lo/8, Razz, Seven Card Stud and Seven Card Stud Hi-Lo Eight or Better are played in rotation.
No set of rules for poker covers every possible irregularity. Most try to cover the most common mishaps, and leave it to the house, or the dealer in a home game, to resolve other problems as fairly as possible with the minimum disruption to the game. Here are some general principles.
"Action" is a pass, bet, check, raise or fold by any player. If two or more players have acted, that constitutes "significant action". Once the initial cards have been dealt and significant action has taken place, the play must continue.
Problems with the deal
Under strict rules, any error in the initial deal, such as giving too many or too few cards to any player, dealing an extra hand or missing out a player who should have been dealt a hand, omitting the shuffle or cut or exposing cards counts as a misdeal, provided that it is pointed out before there has been significant action. In this case the cards are thrown in, the shuffle and cut are repeated and the cards are redealt by the same dealer.
In an informal home poker game, players may agree that redealing in such a case wastes too much time. In that case the deal may be corrected if possible in a way that is fair to the players. For example a player who has one card too few may be dealt another card, a player who has one card too many may hold his cards face down while another player removes a card from it at random, and this card is shuffled into the deck, and so on.
In some home games, if a player misdeals more than a certain number of times in succession (say more than twice), the deal passes to the next player, and the misdealer may be required to pay a penalty, such as matching the pot.
After significant action has taken place, the cards can no longer be redealt. Players should check at the start that they have the right number of cards. A player who has the wrong number of cards at the showdown cannot win the pot.
Players are responsible not only for making sure that they have the right number of cards, but also for ensuring that their concealed cards cannot be seen by any other player, and that their cards are kept separate from any common table cards and especially from the discard pile or "muck". The strict rule is that any hand that touches the muck is dead and the owner can no longer win the pot. In fact "mucking" a hand by placing it in contact with the discard pile is a commonly used as a method of folding.
Players must not deliberately expose cards that are meant to be concealed. Any card accidentally shown to any player (either from the deck or another player's hand) must immediately be shown to all players.
Action out of turn
Players must be careful not to indicate what their action (check, raise, fold, etc.) will be before their turn. If any player accidentally does this, then they are committed to take that action when their proper turn comes.
Cheating and collusion
Players must not advise one another, and non-players are not allowed to help the players in any way. Each player must play alone, in his or her own interest only. Playing in such a way as to help another player is known as collusion. It is considered a form of cheating and would be grounds for being expelled from a formal game.
A very detailed set of rules covering playing procedure, how to deal with irregularities, and so on, can be found on Bob Ciaffone's web site. He provides comprehensive rules for card rooms and for home games.