Koi-Koi

Koi-Koi is a popular card game in Japan played with hanafuda. The phrase "koi-koi" means "come on" in Japanese which is said when the player wants to continue the round.
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 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.

Deal

An initial dealer called the oya is decided upon when the game starts. This can be done with any method the players agree upon. A hanafuda-specific method involves drawing a random card: 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. The other player is called the ko.
To deal, the oya deals eight cards to the ko, then to the table, and then to themselves, though this is normally done with two or four cards at a time. The rest of the cards are set aside as a draw pile, and then the game begins starting with the oya.

Play

On a player's turn, they may match by suit any one card in their hand with one on the table and take both into their point pile. If they cannot match a card from their hand, they must discard a card face up to the table. After matching or discarding a card, they then draw one card from the draw pile and place it face up on the playing area. If this card matches any card now on the table, they must match that card and take both for their point pile; otherwise, it becomes part of the table. If the played card matches two cards on the table, the player gets the choice as to which card to match and therefore keep in addition to the played card. In the event that three cards of a same suit are on the table and a player plays the fourth one, they collect all cards of that suit and take them into their point pile.
After a player's turn ends, if they made at least one yaku that turn, that player must then make a choice. They may end the hand and add the value of their yaku to their point total, or they can choose to continue playing 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 their opponent is able to form a yaku before the caller forms another, the opponent gains double their 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 their opponent had called koi-koi, they get both doubling bonuses for a total of four times their score.
The player who won the last 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, the oya gains one point, and the next hand begins with the same oya. Generally, play continues for 12 hands, with each hand representing a month in the year, 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.

Special cards

The Sake Cup card 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.

''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 to increase the chance of getting combos. Below is a list of special combinations with example and their respective point values. Different rulesets give different point values and some do not include the tsukimi-zake and hanami-zake ''yaku.
Some yaku can be considered extensions of others. For example, one may qualify for both Akatan and Tanzaku by having the Akatan combination as well as two additional five-point cards. In these cases, players are awarded points for both Akatan and Tanzaku.
Points Combo Name Card combo Example
10 Gokō "Five Lights" All five 20-point cards
8 Shikō "Four Lights" All 20-point cards except Ono no Michikaze
7 Ame-Shikō "Rainy Four Lights" Any four 20-point cards, one of which is Ono no Michikaze
5 Sankō "Three Lights" Any three 20-point cards excluding Ono no Michikaze
5 Tsukimi-zake "Moon Viewing" Moon and Sake Cup. Cumulative with Hanami-zake.
5 Hanami-zake "Cherry Blossom Viewing" Curtain and Sake Cup. Cumulative with Tsukimi-zake.
5 Inoshikachō "Boar, Deer, Butterflies" Boar, Deer, and Butterflies
1 Tane Any five 10-point cards, i.e. any combination of animal cards, Eight-plank bridge, and Sake Cup. One additional point is awarded for each additional 10-point card.
10 Akatan Aotan no Chōfuku "Red Poetry and Blue Tanzaku Combination" All three Red Poetry Tanzaku cards and all three Blue Tanzaku cards
5 Akatan "Red Poetry Tanzaku" All three Red Poetry Tanzaku cards
5 Aotan "Blue Tanzaku" All three Blue Tanzaku cards
1 Tanzaku "Poetry Slips" Any five 5-point cards, all of which are Tanzaku cards. One additional point is awarded for each additional 5-point card.
1 Kasu'' Any ten 1-point cards, all of which are normal cards, also known as Chaff cards. One additional point is awarded for each additional 1-point card.

Instant wins and re-deals
There are two special yaku such that, if a player is dealt them before play begins, they are 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 Being dealt four cards of the same suit.
6 Kuttsuki Being dealt four pairs of cards with matching suits.
In popular culture
Nintendo's Clubhouse Games and Clubhouse Games: 51 Worldwide Classics feature Koi-Koi. Other video games which include Koi-Koi include the Yakuza and Sakura Wars franchises.
In the 2009 anime film Summer Wars, characters play Koi-Koi.
In the tenth volume of Kakegurui Twin, Ôri Shimotsukuri plays against Kirari Momobami, the Student Council President, to a game of Koi-Koi.

Points	Combo Name	Card combo	Example
10	Gokō "Five Lights"	All five 20-point cards
8	Shikō "Four Lights"	All 20-point cards except Ono no Michikaze
7	Ame-Shikō "Rainy Four Lights"	Any four 20-point cards, one of which is Ono no Michikaze
5	Sankō "Three Lights"	Any three 20-point cards excluding Ono no Michikaze
5	Tsukimi-zake "Moon Viewing"	Moon and Sake Cup. Cumulative with Hanami-zake.
5	Hanami-zake "Cherry Blossom Viewing"	Curtain and Sake Cup. Cumulative with Tsukimi-zake.
5	Inoshikachō "Boar, Deer, Butterflies"	Boar, Deer, and Butterflies
1	Tane	Any five 10-point cards, i.e. any combination of animal cards, Eight-plank bridge, and Sake Cup. One additional point is awarded for each additional 10-point card.
10	Akatan Aotan no Chōfuku "Red Poetry and Blue Tanzaku Combination"	All three Red Poetry Tanzaku cards and all three Blue Tanzaku cards
5	Akatan "Red Poetry Tanzaku"	All three Red Poetry Tanzaku cards
5	Aotan "Blue Tanzaku"	All three Blue Tanzaku cards
1	Tanzaku "Poetry Slips"	Any five 5-point cards, all of which are Tanzaku cards. One additional point is awarded for each additional 5-point card.
1	Kasu''	Any ten 1-point cards, all of which are normal cards, also known as Chaff cards. One additional point is awarded for each additional 1-point card.