The Show One Show All Rule
A common refrain among poker players who have seen someone show his hand to another player (who presumably has folded) is "show one, show all." This clearly means that since the player has given information to one opponent, all opponents are entitled to the same information. However, in recent years, this rule has been interpreted to mean "show one, show both," meaning if a player exposes one of his cards after the hand is over, the players are entitled to see the other card. In some card rooms, the dealer is then required to turn over the other card.

Reaction to the Show One Show Both Rule
This rule has been met with nearly universal disdain from card players. Poker professional Daniel Negreanu is particularly outspoken about his opposition to this rule. The rule discourages players from showing a single card to confuse their opponents, and most agree that psychology and deception is very much a part of poker. Still, this may be the rule where you play, so you should find out before trying to expose one card.

Exposing a Card to Induce or Diminish Action
Another popular poker rule in card rooms is that you cannot show an opponent a card in order to induce a call or a fold. This rule is adhered to most commonly in tournaments since the actions of one player affect everyone at the table. In NBC's Heads Up Poker Championship, players were permitted to expose a single card, both because it made good television and because no one but the two players involved in the hand would be affected by the action.