Carpentry Tools > Carpenter's square



This L-shaped square ruler is called in various ways: sashigane, kanejyaku, magarigane. The side of the sashigane where the shorter arm (tsumate or tanshi) is on the right side of the vertically placed long arm (nagate or choshi) is called the topside (omote), while the opposite side is called the reverse side (ura). The topside has ordinary scales marked along the arms (omoteme) whereas the reverse side is marked by a scale obtained by multiplying the omoteme by root of 2 (kakume) or by the circle ratio pie (marume).

In addition, though not in use, there exists a scale obtained by dividing 1shaku 2sun (traditional Japanese scales) by 8, marked on the sashigane by letters with portentous significance instead of numbers (monjyaku or rohanjyaku).

The sashigane is used to measure the length and angles utilising its scales and shape, as well as an aid when drawing lines onto different materials. It was treated with uttermost respect since it was a crucial tool for carpenters: stepping on one was promptly admonished by angry shouts.

They were traditionally made of steel or brass but stainless steel was introduced in early 20th century. Some brass sashigane have the angles reinforced with triangularly shaped steel to avoid unwanted changes in the orthogonal angle (kado-tetsu-iri).

Generally its width is 5bu (15mm) and the thickness is 6rin (2mm) giving the long arm a certain level of elasticity. The scale that is used is usually the traditional shaku-sun scale. The metric scale was temporarily the most widespread due to law enforcement, but after relaxation of these laws, the shaku-sun scale has once again gained widespread usage.

Sashigane part names


  Column : The function of sashigane
Although the exact period in history when the sashigane began to be used is unknown, its presence in a Gokan era relief of the first sovereign in Chinese mythology, Fu Xi, found in a rock chamber in Bushishi, suggests its origination in ancient times. It is easy to imagine that the tool traveled over the sea to Japan together with the architectural technology of the continent, in a similarly early period. Pre-modern carpenters used to wrongly believe that the sashigane was invented by prince Shotoku (574-622, regent and politician of the Asakusa period) who was honoured as the ancestral deity.

It is said that the sashigane has 10 different functions. In addition to the most basic functions of measuring length and angles and of aiding the drawing of straight lines, the sashigane can also be used to measure inclinations, to divide up straight lines, and even as a calculator by applying traditional Japanese mathematic theory (wazan). Particularly noteworthy is perhaps the kakume scale, obtained by multiplying the scale on the topside by root of 2, which allows the user to find out the length of the diagonal side of a right-angle triangle without having to calculate it.

This function is highly useful when drawing lines at the corners of buildings. Japanese carpenters elevated its usage to a systematic theory called kiku-jyutsu, which allowed carpenters to deal with complex angles in rooftop corners. Its mastering however was extremely difficult, and the saying "carpenters and sparrows cry in the corners" comes from this.
The mythical sovereign Fu Xi holding a sashigane
(China, gokan era, Bushishi rock chamber)
Contemporary sashigane
(reverse side, made of stainless steel)




 
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