Card games, particularly poker games, may contain one or more cards designated as wild. These may be jokers, or they may be normal ranked and suited cards pressed into wild card duty ("deuces wild" is a common variant). In most cases, the wild card or cards must be agreed upon by all players before the cards are dealt and play commences. There are two common rules regarding wild cards: "fully wild" cards and the "bug".
A card that is fully wild can be designated by its holder as any card they choose with no restrictions. Under this rule, for example, a hand with any natural pair and a wild card becomes three of a kind. With wild cards in play, the best possible hand is a natural royal flush. The common rule in casinos is that a wild card plays as a bug, which is given the rank of ace unless designating it as a different card would complete a straight, flush, or royal flush. Under this rule, a hand such as K-K-Joker-5-2 is just a pair of kings (with an ace kicker), but any four same-suit cards with a bug make a flush, and a hand such as 7-Joker-5-4-3 makes a straight.
There is also a variation of the "Fully Wild" rule in which the wild card (in this instance they are usually jokers as there are traditionally only two and there is only one black and one red) can be any card of the suits matching the cards colour or current suit. For example in a jokers wild game with these rules, the red joker could be used as any card of hearts or diamonds. Inversely, the black joker would be any card of clubs or spades.
Two exceptions to standard poker practice sometimes seen in home games are the double-ace flush rule, and the natural wins rule. The latter rule states that between hands that would otherwise tie, the hand with fewer wild cards wins. This is not common in casinos and should be treated as an exception to standard practice (as is the double-ace flush).
There is a tendency among some players to regard wild cards as "impure" or treat wild card games as silly or amateurish. While it is certainly true that a game with too many wild cards can become so random that all skill is lost, the occasional use of wild cards is a good way to add variation to a game and add opportunities for skillful play. In particular, five-card draw is traditionally played with a joker in California (which plays as a bug), and also plays well with deuces fully wild. Seven-card stud plays well with one or two bugs, especially when played high-low split. Other games such as Texas hold 'em and Omaha hold'em do not play well with wild cards. For some players, the problem with wild card games is that the winner is almost always the hand with the most wild cards, making the other cards irrelevant, and making skill less important.
Another issue with wild cards is that they distort the hand frequencies. In 5-card stud, the stronger hands are less frequent than the weaker hands; i.e., no pair is most common, followed by one pair, two pair, three of a kind, etc. When wild cards are added, the stronger hands gain frequency while the weaker hands lose frequency. For example, if a player holds a pair and a wild card, they will always choose three of a kind rather than two pair. This causes three of a kind to be more common than two pair. But if two pair ranks above three of a kind, the two pair will become more common.