Heian Period Japanese Swords — The Evolution from Straight Swords to Curved Swords

The Age of Straight Swords from the Continent — Nara Period Sword Culture

During the Nara period (710–794), Japanese sword culture was heavily influenced by China and the Korean Peninsula. The swords in use at the time were called "tachi" (大刀) or "chokuto" (直刀)—linear blades with little or no curve, featuring either one or both edges. These were primarily imitations of Tang dynasty (618–907) methods, and most featured ornate mounting (koshirae). They functioned more as ceremonial and status symbols than as weapons.

Swords from the Nara period are still preserved today in the Shōsōin repository, which is attached to Tōdai-ji Temple in Nara. Among these, the "Gold and Silver Inlaid Tang-Style Great Sword" (Kingin-den-sō Kara-tachi) is a straight sword in Tang style lavishly decorated with gold, silver, and mother-of-pearl inlay, transmitted as a relic associated with Emperor Shōmu (701–756). The Shōsōin houses over fifty antique swords and blades, testifying to the level of continental cultural adoption at the time and how swords functioned as symbols of authority in aristocratic society.

Why Did Blades Develop Curves? — Innovation in Metallurgy and Sword-Forging Techniques

The transition from straight swords to curved blades progressed gradually from the late Nara period into the early Heian period. At its core lay the development of Japan's unique iron-smelting and sword-forging techniques.

Of particular note as a transitional stage preceding this technological innovation is the "warabi-te tō" (fern-handle sword). Approximately 250 have been unearthed nationwide, and since their distribution is concentrated in Hokkaido and the Tōhoku region, the theory that they originated in the Kamitsuke area is widely favored. The blades were relatively short at around 40 cm, constructed using the "combined-iron method" (tomo-gane-zukuri), where both the edge and spine were carved from the same iron material. This combined-iron technique was passed on to the subsequent "plucking-hair-style tachi" (kenuki-gata tachi), serving as a midpoint in the morphological transition from continental straight swords to composite-steel tachi. However, opinions still differ on whether the warabi-te tō arose under continental influence (Persian-style curved swords via China and the Korean Peninsula) or developed indigenously within the Japanese archipelago, and no consensus has been reached.

In the Japanese archipelago, a unique smelting method called "tatara seitetsu" (tatara iron-smelting) developed, enabling the production of "tamahagane" (jewel steel) from iron sand. Tamahagane makes possible the creation of blades combining hardness and resilience by combining multiple steels with differing carbon content. The structure of placing hard edge-steel (hagane) on the outside and tough core-steel (shingane) on the inside achieved both sharp cutting ability and resistance to breaking.

A further crucial technological innovation was the establishment of a quenching method called "tsuchi-oki" (clay placement). The swordsmith applies thin refractory clay to the edge portion of the blade and thick clay to the spine side, then heats the entire blade to high temperature before rapidly cooling it in water. As the edge rapidly contracts while the clay-covered spine cools gradually, a natural curve—the "curve" (sori)—develops along the entire blade. The curve was not an aesthetic choice but also a technical necessity born from the pursuit of superior edges. At the same time, this process also produces a pattern along the edge called "hamon" (blade pattern). This pattern would later become central to aesthetic appreciation of Japanese swords.

Mounted Warfare and the Birth of the Tachi — How Combat Changed the Sword's Form

By the middle Heian period (9th–10th centuries), through battles against the Emishi in the northeast and conflicts in the provinces, mounted mobile warfare became Japan's principal combat style. This change fundamentally altered what was demanded of swords.

Straight swords had a physical disadvantage when attacking from horseback. Swinging a linear blade downward tended to result in pushing against the target, failing to transmit sufficient cutting force. In contrast, a curved sword describes an arc as it swings downward, and through "pull-cutting" (hiki-giri)—cutting while drawing the blade—it delivers tremendous cutting force. This was an ideal characteristic for slashing attacks from horseback.

The curved sword that emerged was the "tachi" (太刀). The tachi was worn "suspended" (haku), with the edge facing downward, hanging from the left side of the waist by a leather cord (kawa-o). Unlike the later "inserted" (sasu) style of the uchi-katana and katana, worn edge-up on the waist belt, this suspension method was practical for drawing the longer blade from horseback without difficulty.

The "Kogarasu-maru" (Little Crow), known as one of the oldest surviving curved swords, exhibits a unique shape with curvature from the middle of the blade toward the tip. Now housed by the Imperial Household Agency, this sword is noted by sword researchers as a precious material artifact symbolizing the transition from straight swords to tachi.

During the transition from the late Heian to early Kamakura period, notable changes appeared in the tachi form itself. The characteristic "fukura-gare"—the slightly slender, delicate tip shape—of Heian tachi was replaced during the early Kamakura period by the short, robust "boar-neck point" (inokubi-kissaki). This change corresponded to the practical need to precisely stab through gaps in armor, demonstrating how changes in combat tactics extended even to minute details of the blade.

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

By the late Heian period (11th–12th centuries), regional armed groups—the samurai (bushi)—began to rise to prominence. While the Fujiwara clan controlled court politics, samurai bands charged with maintaining security in outlying regions and guarding manorial estates rapidly gained power. The competition between the two great samurai clans—the Minamoto and the Taira—culminated in the Genpei War (1180–1185), and through major land and sea battles at Ichinotani, Yashima, and Dan-no-ura, the tachi emerged on history's stage as the central player in full-scale combat.

During this period, swords transcended mere weapons for the samurai. Prized swords were passed down as family treasures through generations, and master swordsmiths enjoyed special status and renown. The "Mikazuki Munechika" (Crescent Moon Munechika), attributed to the swordsmith Sanjō Munechika who worked in Kyoto in the late Heian period, is an elegant tachi featuring crescent-shaped nie (hardened patterns) on the blade, and was later counted among the "Five Swords Under Heaven" (Tenka Go-ken). Now housed in the Tokyo National Museum, this celebrated sword transmits to us the aesthetic and technical zenith of Heian sword-forging.

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

The centuries from the Nara through Heian periods were the decisive era when the prototype of the Japanese sword was established. The straight sword from the continent was transformed into a curved tachi through Japan's unique innovations in metallurgy and sword-forging and adaptation to mounted warfare tactics. This evolution was not accidental but a historical inevitability resulting from the complex interplay of three forces: the demands of actual combat, technological innovation, and the rise of the samurai as a new social class.

Between the ornate straight swords sleeping in the Shōsōin and the tachi gleaming amid the waves of Dan-no-ura—separated by merely centuries—lies condensed the great transformation of Japanese history and culture. As of 2026, visitors to the Tokyo National Museum and sword museums throughout the country can view celebrated swords from this era, and through encountering these objects directly, can experience the profound reasons why the Japanese sword has been transmitted as an artwork and spiritual symbol transcending mere weaponry.