Yose (Tatami) 畳寄せ – special wooden trimming used in Japanese Washitsu that comes level with the upper surface of the tatami and serves as a visual cushion between tatami and wall.
The image above shows tatami yose before the tatami is put in. The tatami is laid over the plywood and the upper surface will come flush with the yose. Tatami yose is cut square at the corners, with one butting up against the other, not a miter joint (45 degrees). And as you can see from the image, you can go ahead and screw or nail from the side as you won’t be able to see it once the tatami is in.