Shinogi-Zukuri Blade Geometry: A Complete Guide to the Cross-Section Structure of Japanese Swords

What is "Tsukuri" (Cross-sectional Shape) in Japanese Swords?

"Tsukuri" is the term that describes the cross-sectional shape of a Japanese sword. The cross-sectional form when viewing the blade from a perpendicular angle is determined by its tsukuri, and this shape significantly affects both the sword's functionality (cutting ability, strength, weight balance) and aesthetic appearance.

The majority of modern Japanese swords feature a cross-sectional shape called "shinogi-zukuri," but there is a long historical evolution leading to the establishment of this form. This article focuses on shinogi-zukuri and explains the major types of tsukuri found in Japanese swords and their respective functional significance.

The Structure of Shinogi-zukuri — Nomenclature and Function of Each Component

Shinogi-zukuri is a form of construction in which, when viewing the cross-section of the blade, a ridge line called the "shinogi" runs between the edge and spine, creating a polygonal structure with the blade sloping inward. Centered on this shinogi, each part of the blade is divided as follows.

Components on the edge side (ha-gawa):

• Ha (edge): The portion directly responsible for the sword's cutting ability. Hardened the most through heat treatment.
• Hamon (edge pattern): The pattern created by heat treatment. An important criterion in authentication.
• Ha-machi (edge notch): A cut-in portion at the base of the edge (boundary with the hilt).
• Ji (field surface): The flat surface between the edge and shinogi. The area where the forging pattern appears.
• Jihada (forging pattern): The forging pattern visible on the field surface (itame, masame, mokume, etc.).

The Shinogi Ridge Structure:

• Shinogi (ridge line): The ridge line separating the field and shinogi-ji. An important structural element contributing to blade rigidity.
• Shinogi-ji (upper face): The sloped surface between the shinogi and spine. The area where the blade thickness increases sharply.

Components on the spine side (mune-gawa):

• Mune (spine): The back surface of the blade (opposite of the edge). Various shapes exist such as maru-mune, iori-mune, and kaku-mune.
• Mune-machi (spine notch): The cut-in portion at the base of the spine (boundary with the hilt).

The Function of Shinogi — Why the Ridge Line is Necessary

The existence of the shinogi in shinogi-zukuri carries clear functional significance, not merely aesthetic value.

1. Improved bend strength: By incorporating a rib (protruding ridge line) in the cross-section, bending strength against force increases dramatically compared to a flat surface. This phenomenon can be explained in engineering terms as an increase in the "second moment of area," realizing a structure where a thin blade is resistant to breaking.

2. Improved cutting penetration: By creating a "wedge-shaped" cross-section at the shinogi, resistance decreases after the edge enters the target. As the edge penetrates deeper, the shinogi pushes the target laterally, contributing to excellent cutting performance.

3. Optimized weight balance: By maintaining necessary strength while appropriately distributing the blade's thickness, weight and the center of gravity can be controlled.

Hira-zukuri — The Simplest Structure

Hira-zukuri is a form of construction where the blade cross-section lacking a shinogi consists only of a wedge shape. Commonly seen in small blades (kogata-na), short swords, and wakizashi, it features a structure in which thickness increases uniformly from the edge side to the spine side.

Characteristics of hira-zukuri:

• Simple structure, easier to manufacture than shinogi-zukuri
• Thin blade edge, excellent for thrusting techniques
• Lower bend strength than shinogi-zukuri due to the absence of shinogi
• Often combined with straight edges (suguha)

Representative hira-zukuri short swords: Edo period short swords carried by samurai for self-defense and thinly-forged short swords made as decorative blades for tea ceremony are common examples.

Kiriha-zukuri — Intermediate Form Between Hira-zukuri and Shinogi-zukuri

Kiriha-zukuri is a form of construction in which the majority of the blade maintains a simple cross-section similar to hira-zukuri, while only the blade tip features a different angle. By having the cross-section thin abruptly near the tip (forming a double-edge configuration), this structure enhances cutting ability.

This form is particularly common in blades from the late Sengoku period through the early Edo period and became widespread as a functionally effective construction suited for "cutting attacks against opponents in armor." It was a specialty of swordsmiths from Bizen and Mino, and today a certain number of sword makers still choose this form.

Iori-zukuri and Moroha-zukuri — Specialized Forms

Iori-zukuri: A form where the spine's cross-section tapers to a triangle. The spine's shape resembles an "iori" (iori), or triangular roof, hence the name. Commonly found in naginata.

Moroha-zukuri: A double-edged structure with blades on both the edge side and spine side. Found in straight swords (tsurugi) and common in ancient straight blades.

Blade Shape as Indicator of Period and School

From the type of tsukuri, one can infer the sword's period and place of manufacture (school).

Accurately observing a sword's tsukuri is a fundamental and important skill in authentication, and cultivating the ability to read the period from "shape" as well as from signature and edge pattern is one of the greatest pleasures of Japanese sword appreciation.