The Japanese "Kanejaku" or Shaku Ruler is based on the Japanese unit of the "Shaku," an ancient measure derived from the Chinese "Chi," adopted in Japan in 701, though the units used here are based on those in use during Japan's Edo period, from the 17th to 19th centuries. Its original length was likely much shorter than the standardized 303 mm, as it was originally the length from outstretched thumb to middle finger. Several stories attempt to explain the change, my favorite of which tells about its use in taxation, and its elongation due to corrupt officials attempting to lengthen the unit and therefore increase their take.

The unit standardized in 1891 was based on the "Kanejaku" or carpenter's square measure, as it had seemingly changed the least over time, in part due to its use in temple building. As this is a tool made for (and by) woodworkers, I was inclined to name it after this original unit. The unit was standardized to 10/33 meters, or around 11.93", though its proximity to the foot is happenstance. While the base unit is the "Shaku" of 303 mm (~11.93"), there are further decimal subdivisions. The 寸 or "Sun" is 1/10 of a shaku (~1.19"), and the 分 decimal or "Bu" is 1/100 of a shaku (~0.119").

An interesting aspect of this ruler from a design standpoint was the question of how to represent the numbers and characters of the Japanese language. With the other rulers, I used the characters that were common at the time of the measurement's use, and do not use any modern characters or anachronistic type or language. However in this case, the unit was adopted in Japan in 701 CE, but changed so much over that time that the only reasonable measurement on which to base it was its 1891 standardized length. So, I opted to use the modern numbering scheme, opting out of using more formal Japanese number system or more primitive. As the unit is still in use today among temple builders and other traditional Japanese makers, this is in part a salute to the relevance and steadfastness of the unit itself. I also took the time to design and redraw my own Japanese characters, both for the numbers and the characters for the Bu, Sun and Shaku.

An interesting etymological note- the Shakuhachi instrument of Japan takes its name from its length- one shaku and "hachi" (eight) sun, their combination in a etymological blending being shaku-hachi. Just a fun note.

On another note, there is another "Shaku" unit, called the "Kujirajaku" or Whale Shaku. It is used in the textile industry, much like the yard in the United States. It gets its name from the baleen of a whale, which were used as measurement tools themselves. This ruler is around 379 mm (14.9") inches long, and a great example of using a natural resource with a relatively consistent length as a measurement tool. I'd make a ruler in this size, but its nature as a flexible ruler for cloth would be lost being made of wood, and I'm certainly not interested in acquiring any baleen! There are even more "Shaku" for various trades and special purposes, like the "Gofukujaku" used by traditional Japanese clothiers.

There's some research in English, though not a ton, on the Shaku. Most interesting is the general use of geometry and measure in Japanese home and temple building. Here are some links:

Japanese FAQ page with a large amount of information on the different units, and a brief history on the units.

Tools from Japan's page on the sizing of Japanese Kanna or handplanes, with an interesting big of information on why the "Kujirajaku" is used, instead of the standard "Kanejaku."

Reibo, a website about Shakuhachi including some interesting musicological/technical writing on the use of shaku in determining pitch in the traditional flute.