Shinogi-zukuri Blade Shape and Function — A Thorough Explanation of the Cross-Sectional Structure of Japanese Swords

What Is the "Zukuri" of a Japanese Sword?

The word "zukuri" refers to the cross-sectional shape of a Japanese sword. When the blade is viewed in cross-section, its shape is determined by the zukuri, and this shape has a major influence on both the sword's function (cutting ability, strength, and weight balance) and its aesthetic appearance.

The great majority of Japanese swords made today have a cross-sectional shape called shinogi-zukuri, but it took a long historical evolution before this form was established. This article explains shinogi-zukuri as its central focus, along with the main types of zukuri found in Japanese swords and the functional significance of each.

The Structure of Shinogi-zukuri — Names and Roles of Each Part

Shinogi-zukuri is a construction in which, when the cross-section of the blade is viewed, a ridge line called the shinogi runs between the cutting edge and the mune, giving the blade a multifaceted structure that slopes inward from the shinogi. Centered on this shinogi, the parts of the blade are divided as follows.

Parts on the edge side (hagawa):

• Ha (cutting edge): The part directly responsible for the sword's cutting ability. It is hardened most by yaki-ire (quenching).
• Hamon (temper line): The pattern produced by yaki-ire (quenching). An important criterion in appraisal.
• Hamachi (edge notch): A notch at the base of the cutting edge, at the boundary with the tsuka (hilt).
• Ji (body): The flat surface between the cutting edge and the shinogi. The area where the kitae-hada (forging grain) appears.
• Jihada (grain pattern of the steel): The forging pattern that appears on the ji surface (itame, masame, mokume, etc.).

Structure of the shinogi (ridge line):

• Shinogi (ridge line): The ridge line dividing the ji from the shinogi-ji. An important structural element that contributes to the rigidity of the blade.
• Shinogi-ji: The sloping surface between the shinogi and the mune. The part where the blade's thickness increases sharply.

Parts on the mune side (munegawa):

• Mune (back of the blade): The back surface of the blade, opposite the cutting edge. Shapes include maru-mune (round), iori-mune (peaked), and kaku-mune (square).
• Munemachi (mune notch): The notch at the base of the mune, at the boundary with the tsuka (hilt).

The Function of the Shinogi — Why Is a Ridge Line Necessary?

The presence of the shinogi in shinogi-zukuri is not merely an aesthetic element; it has a clear functional significance.

1. Improved bending strength: By placing a rib (a projecting ridge line) in the cross-section, the resistance to bending increases dramatically compared to a flat plate. This is a phenomenon that can be explained in engineering terms as an increase in the "second moment of area," and it realizes a structure that resists breaking even in a slender blade.

2. Improved cutting-through: Because the cross-section becomes wedge-shaped at the shinogi, the resistance after the edge has entered a target is reduced. As the edge penetrates, the shinogi pushes the target apart to the left and right, contributing to good cutting-through.

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

Hira-zukuri — The Simplest Structure

Hira-zukuri is a construction in which the cross-section of the blade, having no shinogi, is composed only of a wedge shape. It is seen often in small knives (kogatana), tantō, and wakizashi, and has a structure in which the thickness increases uniformly from the edge side to the mune side.

Characteristics of hira-zukuri:

• The structure is simple and easier to produce than shinogi-zukuri.
• The edge is thin and excels at thrusting.
• Because there is no shinogi, bending strength is lower than that of shinogi-zukuri.
• It is often combined with suguha on tantō and wakizashi.

Representative hira-zukuri tantō: It is common in short tantō that Edo-period warriors carried for self-defense and in thinly made decorative swords produced as tea-ceremony objects.

Kiriba-zukuri — An Intermediate Form Between Hira-zukuri and Shinogi-zukuri

Kiriba-zukuri is a construction in which most of the blade has a simple cross-section like hira-zukuri, but a different angle is applied only at the cutting edge. The cross-section narrows sharply near the edge (forming a double bevel), creating a structure that enhances cutting ability.

This construction is seen especially in swords from the latter part of the Sengoku period through the early Edo period, and it spread as a functional form suited to "slashing attacks against an opponent wearing armor." It was a construction at which the tōkō (swordsmiths) of Bizen and Mino excelled, and even today a certain number of tōshō (swordsmiths) choose this construction.

Iori-zukuri and Moroha-zukuri — Special Constructions

Iori-zukuri: A construction in which the cross-section of the mune comes to a triangular point. It takes its name from the resemblance of the mune's shape to an "iori," meaning a triangular-roofed hut. It is seen often on naginata.

Moroha-zukuri: A double-edged construction in which both the edge side and the mune side are sharpened. It is seen on swords (tsurugi) and is a shape common in ancient straight swords.

Blade Shape as an Indicator of Period and School

The type of zukuri allows inference of the sword's period and place of production (school).

Accurately observing a sword's zukuri is a fundamental and important skill in appraisal, and cultivating the eye to read the period from "shape" — not only from the mei (signature) and yakiba (hardened edge) — is one of the great pleasures of appreciating Japanese swords.