This line marker, called a sumi-tsubo in Japanese, is made of a wooden wheel that holds a line of twisted linen. This linen is fed off the wheel into the well (the hollowed-out section of the body), which is filled with ink-soaked cotton. The linen, now covered with ink, is then drawn out through a hole, where it is attached to a pin. The sumi-tsubo is used to make long and perfectly straight lines: running the string through the well gives it a coating of black which, when carefully and properly positioned, can transfer a line on the wood to be cut.
