The size of tatami mats which are laid there is to be about 1 ken times 0.5 ken, but in fact the size is smaller than that by the width of pillars because tatami mats are laid between them.
そこに敷かれる畳も約1間×0.5間の大きさとなるが、実際には柱と柱の間に配置されるので柱の幅の分だけ小さくなる。