Sashigane and Sori Measurement: The Traditional Drafting Tools Swordsmiths Use to Design Blade Geometry

What is Sashigane? — A Swordsmith's Design Tool

Sashigane is an L-shaped metal right-angle ruler. While modern carpenters and woodworkers also use it, for swordsmiths, sashigane transcends being a mere measuring device—it is a "language of design."

Japanese swords possess a distinctive curve called sori, and the shape, depth, and position of this curve determine the character and beauty of the sword. Sori is not created through modern numerical design (CAD/CAM), but rather through sashigane and the craftsman's intuition.

The longer arm of the sashigane is called the nagate, and the shorter arm is called the tsumade. They are used to measure the dimensions of various parts of the sword and to confirm the amount of curvature (sori).

The Dimensional System of Japanese Swords — Blade Parts Measured with Sashigane

To accurately measure a Japanese sword's dimensions, one must know the names of the parts being measured and the measurement methods.

Hachō (Blade Length) The most fundamental dimension indicating the "length" of a sword—not the straight-line distance from the mune-machi (base of the back) to the kissaki (tip), but rather the length measured along the curve of the spine. The traditional method involves holding the sashigane while measuring along the curve like drawing the string of a bow. Today specialized measuring tools (calipers) are also used, but the standard for authentication remains based on sashigane measurement, expressed in shaku, sun, and bu.

Sori (Curvature) Indicates the "depth" of the sword's curve. The measurement method involves drawing a straight line connecting the mune-machi (base) and kissaki (tip) of the sword, then measuring the maximum distance between that line and the spine (back) of the sword. Using both the nagate and tsumade of the sashigane, one measures the depth of the sori while confirming right angles.

Sori is classified into three types by position:

• Koshi-zori (Waist Curvature): Maximum bend near the base third of the blade length—common in tachi from the early Kamakura period
• Naka-zori (Center Curvature): Maximum bend near the center—from the mid-to-late Kamakura through Nanboku-chō periods
• Saki-zori (Tip Curvature): Maximum bend near the tip third—common in large, imposing blades from the Nanboku-chō to Muromachi periods

Moto-haba (Base Width) and Saki-haba (Tip Width) Moto-haba is the width of the blade near the mune-machi; saki-haba is the blade width near the yokote-suji (boundary between tip and blade). The ratio of saki-haba to moto-haba determines the impression of "funbari" (a grounded, stable stance). When the base width is broad and the tip width is narrow, the entire sword acquires "waist strength" and "repose."

Kasane (Thickness) The dimension indicating the thickness of the blade. The thickness at the spine is called "mune-gasane," while the thickness near the center of the blade is called "chū-gasane." If the kasane is too thin, the sword breaks easily; if too thick, it becomes heavy and difficult to handle.

The Swordsmith's Design Process — Dimensional Design Using Sashigane

Let us trace how a contemporary swordsmith uses sashigane when designing a new sword.

1. Confirming Client Specifications and Establishing Design Direction When a client requests "uchigatana, hachō of 2 shaku 3 sun, koshi-zori, base width of 1 sun or greater," the swordsmith first uses the sashigane to confirm whether "those dimensions constitute a viable sword." The relationship between blade length and sori, the balance between base width and kasane—through experience, proportions are known to work, and the swordsmith judges while holding the sashigane: "with this much sori, this width would be too heavy."

2. Creating the Kata (Template) Many contemporary swordsmiths create a "kata" (template)—the sword's outline transferred to a wooden board or thick paper—and then shape the material (tamahagané and iron) to match the template. Sashigane is most active during the template creation phase, precisely transferring the position and depth of the sori.

3. Dimensional Verification During Forging During the forging process (initial drawing → building up → small forging), the sword's form changes with each heating and hammering cycle. Using the sashigane, one confirms that the sori amount, hachō, and moto-haba are approaching the design values. Since Japanese swords change shape before and after hardening, the knowledge to calculate the "correct values before hardening" accounting for post-hardening contraction is essential.

4. Final Measurement After Hardening After hardening (water quenching), the blade's shape changes subtly due to temperature variation. This change is measured with the sashigane, and the final form before polishing is recorded. The measurement values noted in the authentication certificate after completion are those taken by the polisher after final polishing.

Measuring Tools Other Than Sashigane

Here are several of the swordsmith's measuring tools used in combination with sashigane.

Monosashi (Measuring Rod) A specialized bamboo or iron measuring rod for blade length measurement. Bearing markings in shaku, sun, and bu—where 1 shaku = 30.303 cm, 1 sun = 3.03 cm, 1 bu = 3.03 mm. Contemporary swordsmiths also use metric measurements in millimeters, but certificates use the traditional shaku-kanpo notation as standard.

Sori-jōgi (Curvature Ruler) A specialized tool for measuring the depth of sori. It combines a straight section that contacts the spine (back) of the sword with a dial gauge indicating the deepest point of the curve, allowing sori measurement to ±0.1 mm precision.

Nōgisu (Calipers) A tool for precise measurement of base width, tip width, and kasane. Contemporary swordsmiths often use digital calipers, allowing measurement to 0.01 mm precision.

Oral Tradition — "Intuition" That Cannot Be Quantified

Measuring technique is an important pillar of Japanese sword design, but what swordsmiths emphasize most is "sensory judgment that cannot be expressed in numbers."

"The feel when measuring sori with sashigane," "the center-of-gravity sensation when holding the sword after hardening," "the beauty of the 'flow' in the sword's stance after polishing"—these cannot be reduced to numbers. Edo-period secret manuals contain descriptions like "sori of ○ bu is ideal," but the reason *why* that sori is ideal could only be transmitted through sensation.

Contemporary master swordsmiths measure hundreds of their teachers' works with sashigane, and by analyzing averages and deviations themselves, they attempt a "quantification of the master's intuition." Yet ultimately, they say that more than ten years are required before one can possess the conviction that "this is the correct form."

Sashigane is a simple tool, but behind how it is used lies the accumulated experience and observation of craftsmen spanning hundreds of years.