A card is considered dead if it cannot be dealt from the remaining deck. This concept is important when a player is counting the number of outs they have to make their hand, and hence the pot odds the player is being laid to call a bet. If a card is dead, it cannot be counted in their outs.
Cards become dead when they are exposed, either through accident (a player mucking their hand and having it accidentally turn face up) or through the normal course of play (a card in another player's upcards in a game of Seven-card stud is dead to you, since it cannot possibly be dealt to you from the remaining deck).
Imagine that in a game of Hold 'em you are currently holding the ten and jack of hearts:
and the flop comes
You currently have a gutshot straight draw (any nine delivered from the deck into the community cards will give you a straight), and a backdoor flush draw. Ignoring the backdoor draw for the moment, you have four outs to make your straight: the four possible nines in the deck. That means that you are 43:4 against to catch your straight on the next card (there are 47 of the original 52 cards remaining unseen, and 4 of them would make your straight).
Now imagine that before the action gets to you, someone bets, and two players fold. However, as one of the players folds, they accidentally flip their cards over. You see they held a nine-deuce suited:
The player apologizes, and the dealer, after making sure that every remaining player has seen the exposed cards, mucks them. Now what are your odds to make your straight? You have seen that one of the nines in the deck cannot possibly be dealt out of the remaining deck, since it just got exposed and mucked. Now, you have seen 7 cards of the deck so far (not just five, as you had earlier), so there are 45 cards that you haven't seen, only three of which will make your straight. This means you are now 42:3 against making your straight, a notably worse prospect. You are now only about 14:1 to make your straight, where before the player exposed their cards, you thought you were about 11:1. This alters the pot odds and may affect your action. This is why exposing one's cards when you muck, while other players are still in the hand, is against the rules in poker.
To carry this example to extremes, let's also imagine another player gets mad and slams their cards to the ground before storming away. The dealer once again ensures everyone can see their cards, and this time you see they had
Putting aside for the moment the question of why the player would have been so upset at this hand, you now see that two of the nines you would need for a straight are gone, as well as two of the hearts you would need for your backdoor flush (and, for that matter, your only shot at a straight flush, unlikely as it would have been). With two of your outs dead, you now know you are 41:2 to make your straight, or worse than 20-to-1 against.
Ways Cards Become Dead
Cards become dead in a number of ways:
- In Seven-card stud or any game where players hold upcards, all other players' upcards are dead to you (they cannot be dealt to you).
- If the dealer accidentally exposes a card while dealing, they will usually take that card and declare it to be the next burn card, then continue dealing. In this case, the exposed card is dead.
- If a player accidentally or purposefully exposes their cards, they are dead for purposes of your calculations.