The Establishment of the Heian Tachi Tradition — Why the Form Changed from the Straight Swords of the Kofun Period to the Curved Tachi of the Heian Period, and Its Background

The Age of the Straight Sword — The Sword Culture That Arrived from the Continent in the Kofun Period

In the Japanese archipelago during the Kofun period, the chokutō (straight sword), transmitted from the continent, was the dominant weapon. Its form was literally straight, made on the basis of metallurgical techniques from China and the Korean Peninsula. Looking at excavated examples, the mihaba is relatively wide and the blade is forged thin and uniform, representing a versatile design capable of both cutting and thrusting.

Continental chokutō were passed primarily to the ruling class of the Yamato royal authority as ceremonial and prestige objects. Alongside the bronze-vessel culture that continued from the Yayoi period, iron chokutō gradually began to be reproduced within the archipelago. Many chokutō excavated from burial mounds were decorated using inlay techniques, showing that they served both as weapons and as symbols of authority.

However, as Japan passed through the Nara period and entered the Heian period (794–1185), the form of swords changed dramatically. The tachi — bearing the sori (curvature) unique to Japan — was born, and the existing chokutō culture was supplanted. Why did Japanese swords undergo such a fundamental transformation in form? Behind it lay the convergence of three currents: military, technological, and cultural.

The Spread of Cavalry Tactics and the Inevitability of Sori

The shift in the style of combat during the Heian period was a major factor that determined the form of the sword. Before the Nara period, combat centered on infantry-based group fighting, but as the Heian period began, mounted warriors rose to prominence, particularly in the eastern provinces, and the protagonists of the battlefield moved to mounted warriors.

The blade shape required for cutting down from the back of a galloping horse is entirely different from that required for thrusting from the ground. When cutting at an opponent from a charging horse, a straight blade tends to stop the moment the tip enters flesh. A curved blade, on the other hand, allows the edge to slide in continuously during a pulling cut, enabling a deep slash with a single blow from horseback.

From a physical standpoint as well, sori carries important significance. With an appropriate sori in the blade, the movement of the center of gravity when the sword is swung traces an arc, allowing centrifugal force to be efficiently concentrated at the tip. It is also considered that since impact is distributed throughout the blade, there is a structural advantage of being less prone to breakage compared to chokutō.

Furthermore, cavalry combat required the complex action of resting one's weight on the stirrups while managing the reins with one hand and wielding the sword with the other. Consequently, a form that was easy to handle with one hand and could be quickly drawn back after a downswing — a design in which sori concentrates at the sections called koshi-zori and saki-zori — came naturally to be sought.

Innovation in Metallurgical Technology — The Establishment of Tamahagane and Yaki-ire

Alongside the transformation in form, the technology for smelting and tanren (forging) iron also advanced greatly. This was the technological foundation that made it possible to produce tachi with sori.

In the age of the chokutō, iron refining was carried out at relatively low temperatures, and it was difficult to control carbon content. As a result, blades with uniform hardness throughout were mainstream. This was a simple method of production, but it could not escape the trade-off that a blade too hard would break easily, while one too soft would not hold an edge.

One of the innovations the Japanese tōkō (swordsmiths) achieved in the Heian period was the stable production of tamahagane. The tatara (traditional smelting) technique — in which iron sand is smelted at low temperatures over an extended period — was refined, making it possible to sort and selectively use steels with differing carbon content. The composite structure, in which hard steel is placed at the edge and soft steel at the core, also emerged because the technology for sorting materials was established during this period.

The most important technological innovation was the establishment of yaki-ire (quenching). This process of rapidly cooling the heated blade in water dramatically increases the hardness of the edge. At the same time, the difference in cooling between the spine and the edge is said to be one of the factors that causes sori in the blade. In other words, the sori of the tachi also emerged in part as a byproduct of advanced yaki-ire technique, and the evolution of technology drove the transformation in form.

The hamon (temper line) produced by yaki-ire also diversified from this period. The pattern of light appearing at the boundary between the edge and the jigane (steel of the blade surface) vividly demonstrates the skill of tanren, and would later become an important criterion for aesthetic evaluation.

The Dissolution of the Ritsuryo System and the Rise of the Warrior Class — Social Background

In understanding the change in the form of swords, the transformation of the social structure of the time cannot be overlooked. The sakimori (frontier guards) and gundan (provincial military units) systems established under the ritsuryo state system gradually lost their function from the late Nara through the early Heian period. As it became difficult for the central government to mobilize military forces directly, powerful local figures began to organize private armed groups.

The birth of these "warriors" fundamentally changed the demand for and role of swords. The concept of "the warrior's sword" — carried not as a tool in an armory managed by the ritsuryo state, but as something a warrior personally possessed as his own identity — was born.

The sword was elevated from a mere weapon to something embodying lineage, honor, and spirituality. This change accelerated the aesthetic refinement of the tachi. The sori of the blade, the grain of the jigane, and the design of the hamon became signs indicating the owner's status and aesthetic sensibility. Even in the aristocratic society of the Heian period, a fine tachi held great value as an offering to the imperial court or as a gift, establishing a cultural status that transcended the bounds of a mere practical implement.

The Establishment of Yamashiro-den and Yamato-den — The Birth of Regional Schools

From the middle to late Heian period, tōkō schools with their own traditions of sword-making formed in various regions. Among these, the Yamashiro-den of Yamashiro Province (present-day Kyoto Prefecture) and the Yamato-den of Yamato Province (present-day Nara Prefecture) are known as important schools when considering the origins of the Japanese sword.

The tachi of both traditions became the cornerstone that later swordsmiths took as a model under the name kotō (old swords).

Many of the regional traditions that were systematized as the Gokaden (five traditions) after the Kamakura period have their roots in the regional kaji (smithing) culture of the Heian period. The origins of sword-making in Bizen Province (present-day Okayama Prefecture) are also said to trace back to this period, and throughout Japan, groups of tōkō with their own materials, techniques, and aesthetic sensibilities were forming.

From Chokutō to Tachi — Significance as a Cultural Turning Point

The change from the chokutō of the Kofun period to the tachi of the Heian period was not merely a change in the shape of a sword. It was an event that reflected the transformation of Japanese society itself.

It was no coincidence that Japan in the Kofun and Nara periods — when it was absorbing continental culture — used chokutō derived from China and the Korean Peninsula. The birth of the uniquely Japanese tachi in the Heian period is itself the process by which Japan reinterpreted foreign culture to suit its own climate and needs, and created its own culture. It could be said to be the manifestation of "kokufu culture" in the history of swords.

Military necessity drove technological innovation; technological innovation gave birth to new aesthetic standards; aesthetic standards formed the identity of the warrior — this chain gave birth to the Japanese sword as an unparalleled cultural artifact. Many of the great tachi that have been handed down to the present day reached the completion of their form from the late Heian through the early Kamakura period, and the subsequent development of the Japanese sword is nothing other than variations and refinements upon the prototype established in this period.

When viewing a sword, the depth of its sori and the form of its hamon alone bring into relief the background of the period and region in which it was made. From chokutō to tachi — knowing the significance of that transition is the very first step toward a deeper understanding of the Japanese sword as a work of art.