Kai-oke containers come in various shapes and sizes and were used to store kai-awase shells for the traditional 'shell-matching' game. It was very popular from the Heian period (794-1185) to the end of the Edo era (1868). The shells were from a species of clam called hamaguri which had the unique characteristic that none other than the original pair would match together. 360 shell-halves would be divided betweeen two groups of players and each side would put forth one shell and when the opposing player would find a shell to match his he won that shell.