Koi-Koi (こいこい) is a popular card game in Japan played with Hanafuda cards.
The object of the game is to form special card combinations called "yaku" from cards accumulated in a point pile. Players can gain cards in their point piles by matching cards in their hands, or drawn from the draw pile, with cards on the table. Once a yaku has been made, a player can stop to cash in points, or keep going (referred to as "koi-koi", hence the name of the game) to form additional yaku for more points. The point values assigned to individual cards have no effect on the score, but they are helpful to judge their value in forming yaku.
An initial dealer (called the "oya", or "parent"), is decided upon when the game starts. This can be done with any method (rock-paper-scissors, dice roll) the players agree upon. A hanafuda-specific method involves random card draw: each player draws a single card; the player who draws a card from the earliest month is the oya. If both players draw a card from the same month, the player with the higher value card becomes the oya.
To deal, the oya deals eight cards to his opponent (face down), the table (face up), then to himself (face down), though this is normally done two or four cards at a time. The rest of the cards are set aside as a draw pile, and then play begins starting with the oya.
On a player's turn, he may match by suit (i.e. month or flower) any one card in his hand with one on the table and take both into his point pile. If he cannot match a card from his hand, he must discard a card face up to the table. After matching or discarding a card, he then draws one card from the draw pile and places it face up on the playing area. If this card matches any card now on the table, he must match that card and take both for his point pile; otherwise, it becomes part of the table. In the event that the drawn card matches more than one card on the table, the player gets the choice as to which card to match and therefore keep in addition to the drawn card.
After a player's turn ends, if he made at least one yaku that turn, that player must then make a choice. He may end the hand and add the value of his yaku to his point total, or he can choose to continue playing (calling "koi-koi") in an effort to gain more points. In some versions, a player may only call koi-koi once per hand; in others, a player may call koi-koi on multiple yaku-forming turns. Calling koi-koi leaves the player vulnerable, as if his opponent is able to form a yaku before the caller forms another, the opponent gains double his score and the caller earns nothing. If a player has yaku totaling 7 or more points when the hand ends, that player earns twice that value. If a player hits 7 or more points and his opponent had called koi-koi, he gets both doubling bonuses for a total of four times his score.
The player with the most points at the end of the hand becomes the new oya, and a new hand is dealt. Should both players ever run out of cards to play without having formed a yaku on the last play, no points are awarded to either player, and the next hand begins with the same oya. Generally, play continues for 12 hands, but the players can decide to play for a different number of rounds before the game starts.
Some groups, when gambling, will require the player whose score was multiplied to pay a proportionately larger amount of the winnings (i.e. a player who continued play twice would pay twice as much as the other player, since his score would have been doubled).
Special cards 
The Sake Cup is unique in that, though technically classified as a 10-point card, it counts as both a 10-point card and a 1-point card at the same time. Some rules allow the Sake Cup to count as a 10-point card and two 1-point cards at the same time.
Additionally, November's 1-point card, the Lightning, is sometimes used as a wild card that can match any card in some games.
Yaku listing 
If certain combinations of cards are won during play, extra points apply. Sometimes, players will be paired across the table when this rule is used (in multi-player Koi-Koi) to increase the chance of getting combos. Below is a list of special combinations with point values.
Some yaku can be considered extensions of others. For example, one may qualify for both Akatan and Tan by having the Akatan combination as well as two additional five-point cards. In these cases, players are awarded points for both Akatan and Tan (only being the yaku with highest value if the players decide so).
|Points||Combo Name||Card Combo|
|5||Sankō (三光)||Any three 20-point cards excluding the Rainman (Willow's 20-point card).|
|8||Shikō (四光)||The four 20-point cards which exclude the Rainman (Willow's 20-point card).|
|7||Ame-Shikō (雨四光)||Any four 20-point cards including the Rainman (Willow's 20-point card).|
|10||Gokō (五光)||All five 20-point cards.|
|5||Inoshikachō (猪鹿蝶)||The Boar, the Deer, and the Butterflies (the 10-point cards from Clover, Maple, and Peony, respectively). One additional point is awarded for every additional 10-point card.|
|1||Tane (タネ)||Any five 10-point cards, such as animals, the Eight-planked Bridge (Iris), or the Sake Cup (Chrysanthemum). One additional point is awarded for every additional 10-point card.|
|5||Akatan (赤タン)||All 3 Red Poetry Ribbons (found in Pine, Ume, and Sakura). One additional point is awarded for every additional 5-point card.|
|5||Aotan (青タン)||All 3 Purple/Blue Ribbons (found in Peony, Chrysanthemum, and Maple). One additional point is awarded for every additional 5-point card.|
|10||Akatan, Aotan no Chōfuku (赤短・青短の重複)||All 3 Red Poetry Ribbons and all 3 Purple/Blue Ribbons: the combination of Aka-tan and Ao-tan. One additional point is awarded for every additional 5-point card.|
|1||Tan (タン)||Any five 5-point cards, which includes all Ribbons. One additional point is awarded for every additional 5-point card.|
|5||Tsukimi-zake (月見酒)||The Moon and the Sake Cup (Pampas' 20-point card and Crysanthemum's 10-point card). Cumulative with Hanami-zake.|
|5||Hanami-zake (花見酒)||The Sakura Curtain and the Sake Cup (Sakura's 20-point card and Crysanthemum's 10-point card). Cumulative with Tsukimi-zake.|
|1||Kasu (カス)||Any ten 1-point cards, which are all normal (or literally, "junk") cards. One additional point is awarded for every additional 1-point card.|
Instant wins and redeals 
There are two special yaku such that, if a player is dealt them before play begins, he is immediately awarded points. Play then ends before it starts, and the game continues to the next hand. If either of these combinations are dealt to the table, however, the hand is declared void and a redeal occurs. These two combinations are as follows.
|Points||Combo Name||Card Combo|
|6||Teshi (手四)||Being dealt four cards of the same suit.|
|6||Kuttsuki (くっつき)||Being dealt four pairs of cards with matching suits.|
Popular culture 
InoShikaCho is a creature in the anime Dragon Ball, based on the game of Koi-Koi. Inspired by the card combination of the same name, InoShikaCho is part boar, part deer, and part butterfly.
In the anime Kamichu!, Ino, Shika, and Chou are the three tiny spirits that the God Association sends to Yurie as assistants. Their names are puns, as they were given by Yurie for unrelated reasons but correspond to the animals that they resemble, as well as being a card combination.
Even in the anime Ranma 1/2 in the episode no. 124 & 152 Inoshikacho is shown as rivals from the martial arts tea school.
Another reference is team 10 from Naruto consisting of the members Ino Yamanaka, Shikamaru Nara, and Choji Akimichi, as well as the team formed by their respective fathers (and the rest of their families), Inoichi Yamanka, Shikaku Nara, and Chouza Akimichi. The combination is referred to as the InoShikaCho formation.
In the popular 2009 Japanese animated movie Summer Wars, Hanafuda Koi-Koi is played by all members of the Jinnouchi clan, who were taught the game by the head of the clan, Sakae. The game is featured extensively in the movie, especially during the climax.
Pakarnian, John, "Game Boy: Glossary of Japanese Gambling Games", Metropolis, January 22, 2010, p. 15.