Japanese Ink Lines(Sumitsubo) The Sumitsubo is one of the most important marking tools used by the master Japanese carpenter. Similar in function to the Western chalk line, it is used to mark thin, straight lines on wood and other materials. A fine silk line is drawn out through an ink soaked wad of cotton, tensioned and then snapped against the workpiece. This allows you to mark very long (up to 10 m), fine and exceptionally accurate lines. Can also serve as a plumb line.