Koi Koi is a 2-player Japanese card game in which you basically match up cards of the same suit. The goal of the game is to form card combinations that can earn you Yakus or points that can win you the game. It's surprisingly simple once you get the hang of it.
The game is played using a Hanafuda deck. A Hanafuda deck consists of 48 different cards. There are 12 suits within it and 4 per suit. Each suit represents a different month of the year as well as the particular flower at bloom during that time of year. Each card is illustrated with the flower for that suit and may, as well, include other things like objects, animals, or humans as shown below:
Besides a difference in suit and flower, each card has a point value. A card may have a point value of 1, 5, 10, or 20:
* Note that the Sake Cup is a special card. It is a 10-pt card as well as a 1-pt card.
Cards are first handed out, 4 to the dealer, 4 to the other player, and 4 to the table faced-up. Another 4 to the dealer, other player, and table again. Each should now have 8 cards. The remaining cards are place face down in a single pile on table next to the ones face-up. That pile is the draw pile. Here's how it looks like on screen:
Dealer is usually determined by dice-roll or by picks of cards. Whoever has the highest number or card value becomes dealer.
If there are 3 cards of the same suit on table, those three are piled into a single stack. It is taken by the player that discards or draws the 4th card of that suit.
Ending score is doubled for each 20-pt card on opening table.
If any player has 4 cards of the same suit or has 4 pairs (by pair, I mean 2 cards of the same suit) for their opening hand, that player wins the round and a new round begins.
If the cards faced-up on table contains 4 cards of the same suit or 4 pairs, the round is stopped. No one wins and the cards are redealt.
You do two things during each of your turns:
After that, your turn ends and the opponent's turn begins. She does the same as you. Once she's done, it's your turn again. The process is repeated until someone wins or when there are no more cards in hand to be played.
Look at the screenshot in the "Starting the Game" section. This screenshot here is the end result of what happened after mines and the opponent's first turn. During my first turn, I discarded a Clover 1-pt card and made a pair with the Boar 10-pt card on table. Both of the same month, July. I then draw a card from the draw pile and got a Wisteria 1-pt card. It made a match with the Cuckoo 10-pt card on table. All 4 cards are placed faced-up in front of me and are organized by point value. As you can see, I'm hoping to get Boar, Deer, and Butterflies card combination. More on that later.
As stated, to win the round, you need to form certain card combinations in order to earn Yakus. Once you made at least one, you have the option to stop and win the round, or you can Koi Koi, which is to continue the round in hopes of earning more Yakus. If you continue, then you need to make at least another Yaku if you want to go out or continue once more. Ending score is doubled for each Koi Koi made.
Once you stop, Yakus are rewarded for each card combinations made. Like Fans in mahjong, each Yaku is worth 1 point. Once the final score is calculated, that score is added to the winning player's score, while the same amount is deducted from the loser's score. The match is won when the other player's score is zero or below. If each player still has points in their score after a round, more rounds are played until someone's score drops to zero.
When all cards in hand are played, the round ends in a draw and another round begins.
Similar to mahjong. you need at least 1 Yaku in order to go out and win the round, or simply to continue and earn more. You earn Yakus for each card combination you make. Combinations can be made from as little as 2 cards to as many as 10 or more:
* Yaku worth are based on the scores in Koi Koi Shimasyo 2. Note that scores may differ in other games.
Let me clarify a bit on the "additional" combinations. Say you have ten 1-pt cards. This is worth 1 Yaku. However if you have twelve 1-pt cards, then your cards would be worth 3 Yakus (1 Yaku for the initial ten and 2 more Yakus for the two extras). This is why it is a good reason to continue if you have a winning combination instead of stopping. You can earn more Yakus within another single turn.
You are only scored for one combination in the 20-pt card category. If you have a Dry 4 Bright, for example, you will not earn additional Yakus for making a Dry 3 Bright.