When the shorter ear of the shell faces you, if you turn the umbo toward yourself, dashigai (clamshells which are put in the bucket and later taken out one by one to play kaioi) as yin (passive) is on the right, and jigai (clamshells which are placed on the mat in kaioi) as yang (active) is on the left.
貝は耳の短い方を前にして、頂を自分の方に向けると、右が出し貝すなわち陰、左が地貝すなわち陽である。