Kai-Ooi (貝覆い, ‘shell covering’) is a game played in Japan during the Nara and Heian periods. It is thought to have developed from a completely different game called Kai-awase (貝合わせ, ‘shell matching’), and the game is sometimes mistakenly referred to as Kai-awase.
The game is played using clam shells. Later sets have the undersides of the halves of the shells elaborately painted with gold and colorful imagery. Such sets were considered as luxury items, and are usually used a as dowry of a noble bride, since the matching of shells could be considered a metaphor for marriage. Because of the style of gameplay and also the paintings on the shells, such sets could be considered the ancestor of karuta.
The game is played in similar fashion to the western playing card game “Concentration”, in which the two halves of clamshells are separated, and each player must hold one to find the other. This game is possible because when the two halves of the shell are split into two, they do not split straight down the middle, and so only the two halves of the same shell could fit together.
Prepare 360 clamshells.
The number 360 represents the days of the year.
Each clamshell is divided into two halves:
The ground shells and put-out shells are segregated in two different buckets (貝桶 - kai-oke) for storage.
Any number of players may play the game. The dealer may or may not be one of the players participating in the game.
First, place 12 ground shells face-down on the floor, arranged in a small circle in a way such that each ground shell’s pointed part is facing the middle of the circle.
The number 12 represents the months of the year.
Next, place 19 ground shells encircling the first row of ground shells on the floor.
Each additional row contains 7 more ground shells than the previous row; the number 7 represents the 7 days of the week.
Next, place 26 ground shells encircling the second row of ground shells on the floor.
Next, place 33 ground shells encircling the third row of ground shells on the floor.
Next, place 40 ground shells encircling the fourth row of ground shells on the floor.
Next, place 47 ground shells encircling the fifth row of ground shells on the floor.
Next, place 54 ground shells encircling the sixth row of ground shells on the floor.
Next, place 61 ground shells encircling the seventh row of ground shells on the floor.
Finally, place 68 ground shells encircling the eighth row of ground shells on the floor.
In total, all 360 ground shells are placed face-down on the floor neatly in a circle.
Players sit around the circle.
The dealer then takes one “put-out shell” from the bucket and places it face-down in the middle of the circle.
All players must then check the ground shells surrounding it, and choose which one of them is the correct half of the “put-out shell”.
Once a player announces their decision, they take the “put-out shell” and the ground shell they have chosen, and try to fit them together.
If they fit perfectly, then the player takes both halves of the shell with them. The game continues with the dealer taking another “put-out shell” from the bucket and placing it in the middle of the circle.
If they don’t fit perfectly, then the player returns both shell pieces on the floor where they were originally placed.
Repeatedly making mistakes in matching shells was considered a disgrace among players of the game.
The game continues until all shells have been taken by the players. The player with the most shells wins the game.