Sword Morphology of Early Heian Japan: Koshi-zori, Slender Body, and Ko-kissaki

Early Period Sword Form — How to Read "Formal Characteristics"

When reading the formal characteristics (sword form) of Japanese swords, the main elements that specialists focus on fall into five broad categories.

1. Sori (curvature): The amount of blade curvature and the location of the apex
2. Mihaba (blade width): The ratio between moto-width (width near the base, close to the moto-kasane) and saki-width (width at the tip)
3. Kasane (thickness): The thickness of the blade
4. Kissaki (point shape): The length and form of the tip
5. Form of sori (koshizori, nakizori, sakizori): Where the peak of the curvature is located

The early Heian period (794—late 10th century) marked the very beginning of an era when Japanese swords transitioned from straight blades (chokutou) to curved tachi. The sword forms of this period possess formal characteristics clearly distinct from those of later swords.

Koshizori (Waist Curve) — Characteristic of Early Period Blade Curvature

One of the most striking formal characteristics of early Heian tachi is "koshizori (waist curve)."

Koshizori refers to a curvature in which the peak of the blade's curvature (sori) lies near the nakago (tang), on the base side (waist) of the blade. This is distinguished from "nakizori (middle curve)," where the peak of curvature exists near the center of the entire blade, and "sakizori (tip curve)," where the curvature is biased toward the tip.

Why is koshizori common in early Heian swords? Multiple theories exist, but the most compelling explanation comes from both "the development stage of forging technology" and "practical requirements."

In terms of forging technology, when the refinement of steel through repeated hammering and folding first began, achieving uniform curvature throughout the entire blade was technically difficult. It is believed that naturally curving the blade from the edge base (waist) made it possible to obtain relatively stable curvature.

From a practical standpoint, koshizori was advantageous from the perspective of drawing the sword (nuki-zao) while mounted on horseback. Tachi were worn suspended at the waist with the edge downward (the "tachi carrying method"). When drawing the blade from the scabbard, a sword with koshizori could be drawn along a trajectory that was less likely to interfere with the horse's saddle.

When measuring existing early Heian tachi (joukoutou or early period examples of ancient swords), most exhibit koshizori with their maximum curvature peak at approximately 30—40% of their total length from the nakago.

Slender Base and Tip Width Ratio — The "Tapering" of Early Period Blades

Compared to tachi from the Kamakura period onward, early Heian tachi are characterized by overall slenderness of the blade width, and by a large difference between moto-width (motohaba) and saki-width (sakihaba).

Moto-width refers to the width near the ha-machi (the boundary between blade and tang). Saki-width refers to the width in front of the yokote (the boundary line between the tip and edge). The smaller the ratio of saki-width to moto-width (i.e., the more the blade narrows toward the tip), the more the sword is said to have "large fumigomi" or "strong tapering."

In early Heian tachi, examples with saki-width at 55—65% of moto-width are common. By contrast, tachi from the mid-Kamakura period tend to show saki-width/moto-width ratios rising to 65—75%.

The slender form and strong tapering of this sword shape are attributed to the influence of continental (Tang) sword styles. Because early Heian smiths began with methods and forms of swords transmitted from China and the Korean Peninsula, the slender blade shape of the straight sword period was maintained for some time even after the transition to curved tachi, according to this interpretation.

In one of the oldest surviving tachi, the "Tachi Meiji Furuhouoki" (estimated 9th—10th century) held by the Tokyo National Museum, these three characteristics of slenderness, koshizori, and small kissaki can be confirmed.

Kokissaki (Small Point) — The Restrained Point Form of the Early Period

The third formal characteristic of early Heian tachi is "kokissaki (small point)."

Kissaki refers to the shape of the sword's tip, and there are several types distinguished by size and form. A "small kissaki" is a point that is relatively short, with a restrained fukura-cho (point length) in the portion following the edge. In contrast, the "inokubi kissaki" that developed in the mid-Kamakura period is a short, rounded point, while the long and large "ookissaki" developed from the late Kamakura through the Northern and Southern Courts periods.

Two reasons are cited for why kokissaki is common in the early Heian period.

Constraints of forging technology: The tip experiences intense stress concentration and is one of the technically most difficult areas. In the early period, when precise shaping of the tip was difficult, a small, restrained kissaki may have been the natural form.

Practical characteristics: Kokissaki is considered a form that emphasizes cutting (kiri) over thrusting (tsuki). This aligns with the fact that the primary combat method of the Heian period was cutting blows from horseback.

Hirazukuri and Shinogizukuri — A Period of Coexistence

The early Heian period was also a transitional era in the cross-sectional shape (tsukuri) of the blade.

Before the establishment of "shinogizukuri," the standard form of modern Japanese swords—a construction with a ridge called the "shinogi" running along the center of the blade width—examples of "hirazukuri" also existed, in which the blade's cross-section is nearly triangular without a shinogi.

While hirazukuri is now used primarily for short blades and wakizashi, examples of early period tachi with cross-sections close to hirazukuri are also recognized. This likewise reflects both the influence of continental swordwork and the stage of technical development.

Reading Early Period Sword Form from Surviving Examples

Surviving early Heian tachi are extremely rare, with most designated as Important Cultural Properties or National Treasures and housed in museums and shrines.

Comparing the actual measurements of these examples reveals common formal characteristics: total length of 75—80cm (blade length of approximately 65—72cm), koshizori of approximately 1.5—2.0cm, moto-width of 2.5—2.8cm and saki-width of 1.4—1.8cm (saki-width/moto-width ratio of 55—65%), and kokissaki.

Compared to later swords, these characteristics give an impression of "slender form with curvature biased toward the base," but they were in fact necessary forms perfectly adapted to both the forging technology of the early period and the combat methods of the time.

Conclusion — The Influence of Early Period Sword Form on Later Eras

The prototype sword form established in the early Heian period—"koshizori, slenderness, and kokissaki"—became the foundation upon which improvements and developments were made in subsequent eras. In the Kamakura period, formal changes occurred in which moto-width broadened and the kissaki became inokubi kissaki; in the Northern and Southern Courts period swords grew larger; and in the Muromachi period they shrank and became more practical—all these transitions can be understood through comparison with the early period prototype.

In dating Japanese swords by era, the formal characteristics of sword form are among the most important criteria for judgment. Understanding what form early period swords possessed leads to understanding the "reference points" for deciphering the transitions of later periods.