What is Futasujibi in Japanese Katana terminology?

The term "Futasujibi" (二筋樋, ふたすじひ) refers to a type of groove (Hi) carved into a Japanese sword. This design features two parallel grooves of the same width. The grooves on the sword body not only enhance the structural strength of the sword, but also carry significant meaning as expressions of religious faith. The Futasujibi is intended to represent the Goma-bashi, an incarnation of Fudo Myoo (Acala, the God of Fire). Furthermore, the presence of the grooves gives the sword body a more streamlined appearance, thus playing a role in enhancing the aesthetic beauty of the sword.