Number of Players: 2

Cards Used: 1 full deck of 48 cards.

Game Length: 12 rounds

Goal: To form captured hands faster than your opponent

The dealer deals 8 cards face-down to each player and 8 face-up to the field. He does so by dealing 4 cards at a time, first to his opponent, then to the field, and finally to himself. The following is an example of a Koi-Koi starting position:
There are two dealt hands in Koi-Koi, which should be checked for once hands have been dealt. They are:

Hand of Four
手四
4 cards from the same month
Joined Hand
くっつき
4 sets of 2 cards from the same month

If a player has one of the dealt hands he wins the round and gains 6 points (if both players have one, ignore the hands). Otherwise play begins. Starting with the dealer each player captures cards (as described in the General Rules). This continues until a player forms one of the captured hands listed below or the players run out of cards. If the players run out of cards the round is a draw and no points are scored.

If a player forms a captured hand they have to make a choice whether or not to continue in an attempt to get a higher score. They announce 'Koi!' (Japanese for 'come on!') or 'Game'.

If the player announces 'Game', he wins the round. He gains points for all of his captured hands. If his points for the round are 7 or more, he receives double for the round.

If the player announces 'Koi!', the game continues as before. 'Koi!' is a way for a player to bet that he can form a new captured hand (or improve the existing hand) before his opponent can form his first hand. If the player succesfully improves an existing hand (by going from 10 dregs to 11 dregs, for example) or forms a new hand before his opponent he does not gain any additional points for doing so. If, however, his opponent successfully forms a hand first, the opponent's score for the round is doubled and the player receives no points. The only advantage of 'Koi!' for a player is that it gives him a chance to increase his score. If a player's 'Koi!' bet is successful he must then choose whether to 'Koi!' again or 'Game'.

The following captured hands are used in Koi-Koi:

Captured Hands

Five Lights
五光
10 points
Four Lights
四光
8 points
Rain Four Lights
雨四光
7 points
Rain Four Lights consists of any 3 lights and the Rain card.
Three Lights
三光
6 points
Three Lights can be formed from any 3 of the non-Rain lights.
Note: All of the above hands are mutually exclusive.
Blue Poems
青タン
6 points
Red Poems
赤タン
6 points
Poems
タン
1 point
Any 5 poetry slip cards. Additional slips are +1 point each.
Boar Deer Butterfly
猪鹿蝶
6 points
Animals
タネ
1 point
Any 5 'animal' cards. Additional animals are +1 point each.
Moon Viewing
月見酒
5 points
Flower Viewing
花見酒
5 points
Dregs
カス

1 point
Any 10 'dregs'. Additional dregs are +1 point each.

To sum up, in Koi-Koi the loser's score for a round is 0. The winners score is the value of his captured hands. The winner's score is doubled if his opponent declared 'koi' or if his score is over 7 (quadrupled if both are true).

For example:
If a player had the above captured cards he would score 14 points assuming that his opponent did not declare 'koi'. He would score 7 points for the 'Rain Four Lights' captured hand. That score would be doubled because it is 7 or more.