The planks have knots. This does not affect the performance of the product and is more affordable costwise. Knotty wood is also believed to contain more aromatic oils. When the bathtub is skillfully assembled considering the pattern of the knots, it reinforces the confortable "woody" atmosphere.
The trunk is cut diagonally to avoid knots.The planks exhibit the long, elegant, pure grain of hinoki without imperfections. This cut is the most appreciated in Japan even though the subtle grain becomes more difficult to perceive with the passing of years.