What is Chokutō — Why the Japanese Sword Developed Curvature | The Nara and Heian Periods and the Evolution to the Tachi

What is Chokutō — Definition and Surviving Masterworks

Chokutō refers to ancient straight swords with almost no curvature in the blade. Japanese swords made from the Kofun period through the Nara period and early Heian period are generally of this form, and in sword classification they are called jōkotō to distinguish them from the "curved Japanese swords (wantō)" of the mid-Heian period onward. It is conventional to distinguish in writing between single-edged examples, called tachi (or daitō), and double-edged examples, called tsurugi (or ken).

The following are representative surviving chokutō.

• Kingin Densō Kara-tachi — Shōsōin (Nara). A chokutō in Tang style decorated with gold, silver, and mother-of-pearl inlay, passed down as a relic associated with Emperor Shōmu.
• Shichiseiken and Heishi Shōrinken — Shitennō-ji (Osaka). Chokutō from the Asuka period traditionally associated with Prince Shōtoku, both kokuhō (National Treasures). The Shichiseiken bears an engraved design of the seven stars of the Northern Dipper.
• Suiryūken — Tokyo National Museum. A chokutō of the Nara period transmitted from the Shōsōin, known for having had a new koshirae (mounting) made for it in the Meiji period as the personal sidearm of Emperor Meiji.

The differences between chokutō and Japanese swords (wantō) can be summarized as follows.

The Era of the Chokutō Transmitted from the Continent — Sword Culture of the Nara Period

In Japan during the Nara period (710–794), sword culture was heavily influenced by mainland China and the Korean Peninsula. The swords in use at the time were straight, nearly uncurved blades of either single-edged or double-edged form, known as tachi (or daitō) or chokutō. These were largely modeled on the techniques of the Tang dynasty (618–907), many featuring elaborately decorated koshirae (mountings), and served more as ceremonial and rank symbols than as functional weapons.

The Shōsōin, an imperial repository affiliated with Tōdai-ji in Nara, still preserves Nara-period swords today. The Shōsōin holds more than fifty ancient swords and blades, conveying to us today the level at which continental culture was absorbed at the time and the degree to which swords functioned as symbols of authority in aristocratic society.

Why Did Swords Develop Curvature — Innovations in Metallurgy and Blade-forging Technique

The transition from chokutō to curved swords progressed gradually from the late Nara period through the early Heian period. At the heart of this change was the development of Japan's own distinctive metalworking and blade-forging techniques.

Commanding attention as a transitional stage that preceded this technological innovation is the warabitō (warabi-handled sword). Approximately 250 specimens have been excavated across the country, and because their distribution is concentrated in Hokkaidō and the Tōhoku region, the theory that they originated in the Kamitsukeno region is regarded as the most plausible. The blades are short, around 40 cm, and were made by the tomogane-zukuri method, in which the cutting edge and the mune (back of the blade) are carved from a single piece of iron. This tomogane-zukuri technique was inherited by the subsequent Kenuki-gata Tachi, occupying the midpoint in the morphological transition from continental-style chokutō to the tamahagane composite-structure tachi. The theory that the warabitō emerged under continental influence (via curved swords of Persian origin transmitted through China and the Korean Peninsula) and the theory that it developed indigenously within the Japanese archipelago remain in opposition to this day, with no consensus view established.

In the Japanese archipelago, the distinctive smelting method known as tatara (traditional smelting) developed, enabling the production of tamahagane from iron sand. By combining multiple grades of steel with different carbon contents, tamahagane makes it possible to produce a blade that possesses both hardness and toughness. The structure in which hard hagane (cutting-edge steel) is placed on the outside and resilient shingane (core steel) on the inside achieves both a keen cutting edge and resistance to breakage.

An even more significant technological innovation was the establishment of the yaki-ire (quenching) method known as tsuchioki (applying the clay coating). The tōshō (swordsmith) applies a thin layer of refractory clay to the cutting-edge side of the blade and a thick layer to the mune (back of the blade) side, then heats the whole blade to high temperature before plunging it into water for rapid cooling. At this point, while the cutting-edge side contracts rapidly, the mune side covered in clay cools gradually, so a natural curvature — sori (curvature) — arises throughout the blade. Sori was not an aesthetic choice but also a product of technical necessity in the pursuit of a superior blade. At the same time, this process produces the pattern along the edge known as hamon (temper line). This pattern would also go on to become central to the aesthetic appreciation of Japanese swords in later ages.

Equestrian Tactics and the Birth of the Tachi — Actual Combat Changed the Shape of the Sword

By the mid-Heian period (9th–10th centuries), mobile cavalry warfare had become Japan's principal style of combat, forged through battles with the Emishi of the Tōhoku region and conflicts in the provinces. This change fundamentally altered the functional demands placed on swords.

When attacking an enemy from horseback, the chokutō has a physical disadvantage. Bringing a straight blade down tends to produce a pushing motion at the edge against the target, and cutting power is not effectively transmitted. A curved blade, by contrast, traces an arc during the downstroke, and through hikigiri — the action of cutting while drawing the blade — delivers tremendous cutting power. This was an ideal characteristic for mounted slashing attacks.

The curved sword that came into being in this way is the tachi. The tachi was worn as a suspended sword by hanging it from the left side of the waist with leather cords (kawao), with the cutting edge facing downward. This carrying method, different from the later uchigatana style of thrusting the sword edge-upward through the waistband, was a practical arrangement that allowed even a long blade to be drawn smoothly from horseback.

Kogarasu-maru, known as one of the earliest surviving curved swords, exhibits a distinctive form in which the curvature runs from roughly the midpoint of the blade toward the tip. This sword, now in the collection of the Imperial Household Agency, is regarded by sword researchers as a precious surviving specimen that symbolizes the transitional period from chokutō to tachi.

In the transitional period from the late Heian period to the early Kamakura period, notable changes are also visible in the form of the tachi itself. The fukura-gare characteristic of Heian-period tachi — in which the kissaki (point) is somewhat narrow and slender — was replaced in the early Kamakura period by the short, robust kissaki known as inokubi-kissaki. This was a change linked to the practical demand for accurately thrusting into gaps in armor, demonstrating that shifts in the nature of combat extended even to the finest details of the blade.

The Rise of the Warrior Class and the Deepening of Sword Culture

By the late Heian period (11th–12th centuries), the provincial armed groups known as bushi (warriors) began to rise to prominence. While the Fujiwara clan held the reins of court politics, warrior bands charged with maintaining order on the frontiers and guarding shōen (estates) rapidly grew in power. The rivalry between the two great warrior clans — the Minamoto and the Taira — culminated in the Genpei War (Genpei no Ran) of 1180–1185, and through the large-scale land and sea battles of Ichi-no-Tani, Yashima, and Dan-no-Ura, the tachi emerged on the stage of history as the central weapon of full-scale warfare.

In this era, the sword had become something that transcended a mere weapon for the bushi. Famous swords were passed down as family treasures from generation to generation, and skilled tōkō (swordsmiths) enjoyed special status and renown. Mikazuki Munechika, said to have been forged by Sanjō Munechika — a tōkō active in Kyō in the late Heian period — is an elegant tachi bearing crescent-shaped nie (visible martensite crystals) on the blade, and was counted in later ages among the "Tenka Goken" (the Five Great Swords Under Heaven). This celebrated sword, now in the collection of the Tokyo National Museum, conveys to us today the aesthetic and technical heights reached by Heian-period blade-forging.

Frequently Asked Questions (FAQ)

Are chokutō included in the category of Japanese swords?

In a broad sense, "Japanese sword" can refer generally to all swords made in Japan, but in sword classification it is conventional to use "Japanese sword" for the curved wantō of the mid-Heian period onward — made from tamahagane and featuring sori and shinogi-zukuri — and to distinguish chokutō made before that time as jōkotō.

Was the sword the only weapon of the Heian period?

The principal armament was in fact the bow and arrow. Combat by bushi from the mid-Heian period onward centered on kisha — mounted archery, shooting arrows from horseback — with the tachi serving as a weapon for close-quarters and grappling combat. The naginata also developed as a weapon for infantry.

Where can chokutō be seen today?

At the "Shōsōin Exhibition" held every autumn at the Nara National Museum, Shōsōin swords are displayed in some years. The Shichiseiken and Heishi Shōrinken of Shitennō-ji (Osaka) are sometimes displayed in the temple's treasure hall, and the Tokyo National Museum also holds and exhibits jōkotō including the Suiryūken.

Conclusion — The Period When the Prototype of the Japanese Sword Was Formed

The several centuries spanning the Nara and Heian periods were a decisive era in which the prototype of the Japanese sword was established. The chokutō of continental origin was transformed into the curved tachi through innovations in Japan's own metallurgy and blade-forging technique and through adaptation to equestrian tactics. This evolution was not the product of chance but a historical inevitability in which three forces were complexly intertwined: the demands of actual warfare, technological innovation, and the rise of the bushi as a new social class.

Between the magnificent chokutō sleeping in the Shōsōin and the tachi that gleamed in the waves at Dan-no-Ura — separated by only a few centuries — is compressed a great transformation in Japanese history and culture. As of 2026, the celebrated swords of this era can be seen in person at the Tokyo National Museum, the Japanese Sword Museum, and other institutions across the country, and one can experience firsthand — through the actual objects — the profound reasons why the Japanese sword has been handed down as an art form and spiritual symbol that transcends a mere weapon.