Kai-awase

The original game, using shells, was played by Heian aristocrats who would compose poems about the shape, size, and color of clams, and then compete against each other to see how well they wrote songs.

[3] During the late 12th century, "Kaiōi" was a game in which multiple pieces of clam shells were mixed up, separated from the left and right sides, and players would find a pair among them.

[3][4] In the late 13th century, shells became increasingly elaborate and beautiful for the purpose of play and comparison.

[4] At the beginning, the game only competed for superiority based on the shape, color, size, and variety of shellfish, which were divided into left and right sides.

In later generations, songs were also written separately, with drawings inside a pair of shells.