Reference lines in Japanese carpentry

While studying carpentry at Suikoushya International Craft School, Sensei Takami Kawai repeatedly mentioned the importance of the reference line. The reference line is a single line that goes down the centre of a face of a board. As a furniture maker, the reference line goes against some of my favourite woodworking principles, but the sheer genius of it became clear over the course of the month, and I am very excited to implement it’s use in my work.

Principles of Flat and Square

The way I have been taught woodwork, cabinetmaking, and furniture making, involves first squaring up your stock. You can never trust that a board is flat and square, so the first thing you do is you square it up. All measurements and mark ups are done based on you’re newly flattened board, and only once you have been able to make it perfectly square will any of the rest of your work be successful. If you board isn’t flat, then your box will be twisted, or your joinery won’t come together. As far as I was aware, there was no way to do exact work, the kind of exact work I knew Japan was known for.

Takami Kawai repeatedly reminded us that we cannot trust that the boards we use are flat and square, but he had absolutely no intention of correcting that. In fact, many of the timber pieces used in the construction of his work piece were only roughly hewn and still curved and shaped like the tree it once was (see the image above as an example). Instead of squaring it up, Japanese carpenters rely exclusively on centred reference lines.

The Line

The Sumitsubo is an ink pot with a string built into it and it is used like a chalk line to snap a straight line in ink between two points. The carpenter when marking out a board starts by putting the pin part in the center of one end of a board, holding down the string in the center of the other end, and carefully snapping a long straight line. This becomes the reference line, and all joinery, measurement, or marking of any kind is done based on this single line.

This basic concept allows a carpenter to do precise work with imprecise timber.

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