The Curvature of Japanese Swords: Types and Functional Significance of Koshi-zori, Naka-zori, Saki-zori, and Uchi-zori

What is Sori? The Element That Defines the Shape of Japanese Swords

When discussing the shape of a Japanese sword, sori (curvature) is one of the most fundamental and important elements. Sori refers to the degree to which the cutting edge curves in an arc relative to a straight line along the mune (spine/back) side of the blade. The sori measurement is calculated as the vertical distance from the hamachi (the boundary between the blade and the nakago) to the point of maximum curvature on the blade, and is expressed in centimeters.

The standard sori for a typical uchigatana is approximately 6 bu (about 1.8 cm) to 8 bu (about 2.4 cm). Tachi often have deeper sori, and it is not uncommon for them to have sori exceeding 1 shaku (about 3 cm). The sori measurement is always recorded on authentication certificates, such as those issued by the NBTHK for registered swords.

Swords with no sori—those with the blade and spine nearly parallel and no curvature—are called chokutou (straight swords). Continental-derived swords from before the Asuka and Nara periods were nearly all straight-form, and the distinctive form of the Japanese sword was created when curved tachi were established during the Heian period.

Koshizori (Hip Sori)—The Sori Form Born from Heian Cavalry Warfare

Koshizori is a form of sori in which the point of maximum curvature on the blade is located near the nakago (tang), that is, at the "hip" position. Overall, the blade curves most dramatically near the base and becomes progressively gentler toward the kissaki (point).

Koshizori is a form of sori frequently seen in tachi from the Heian period through the early Kamakura period. It is considered well-suited to the cavalry tactics established during the Heian period—the method of shooting arrows from horseback and then charging in with a tachi. When drawing a tachi from its position worn at the hip on horseback (with the blade worn edge-downward), the sori at the hip is optimized to the mechanics of the blade sliding from the koiguchi (mouth of the scabbard), enabling a quick draw.

Extant tachi from the Heian and early Kamakura periods contain many exemplary cases of koshizori. Representative examples include tachi of the Sanjo and Gojo schools of Yamashiro Province (Kyoto) and tachi of Ko-Bizen of Bizen Province (Okayama). Tachi by Sanjo Munechiha and Tomonari are highly regarded as complete expressions of the Heian sword style, unified with the formal beauty of koshizori.

Nakazori (Mid Sori)—The Blade Style of Kamakura Warriors

Nakazori is a form of sori in which the point of maximum curvature is positioned approximately at the center of the blade. Compared to koshizori, the sori is distributed more evenly across the entire blade, creating a visually more "balanced" curve.

Nakazori is frequently seen in tachi from the mid to late Kamakura period. As cavalry warfare became increasingly sophisticated and opponents became predominantly samurai armed in armor, the functional demands on the blade increased. It is believed that nakazori, with more uniform slicing efficiency across the entire blade and less susceptibility to edge chipping compared to koshizori, became widely adopted as a style more suited to practical combat.

Many tachi and tantou by Masamune (Soshu Kamakura, late Kamakura period) and Sadamune have forms close to nakazori, demonstrating the completion of the Kamakura sword style. Nakazori forms are also frequently confirmed in mid-Kamakura period tachi by Bizen smiths such as Yoshioka Ichimonji.

Sakizori (Tip Sori) and Uchizori (Reverse Sori)

Sakizori is a form of sori in which the point of maximum curvature is positioned near the kissaki (tip), with the blade remaining relatively straight near the base and curving increasingly toward the tip. It is frequently seen in Edo period uchigatana, particularly from the Shintou period onward.

Functionally, sakizori is noted for its compatibility with the way uchigatana are worn with edge upward at the hip (sashikoshihaki). While tachi are worn with the edge downward in a hanging position (haku), uchigatana are worn with the edge upward in a thrusting position (sasu). There is practical reasoning that when worn in this position, sakizori provides an advantageous angle for the kissaki on the initial strike after drawing.

Uchizori (reverse sori) is a form of sori that curves in the opposite direction from normal—the curve faces the blade side (concave) rather than the spine side. While most Japanese swords curve convex on the blade side, uchizori curves convex on the spine side. Uchizori is primarily seen on tanto and yoroitoshi (armor piercers), and is considered a form that allows easier control of the kissaki angle when thrusting into gaps in armor. Examples of uchizori can be found in historical records from certain periods categorized as "sun-nobi no tanto" and "koshimoto no tanto."

Controlling Sori in the Forging Process—The Smith's Technical Judgment

The sori of the blade is created during the forging and quenching (yakiire) processes. During the folding and hammering stages (ori-kaeshi tanren) where iron is folded many times and drawn out, and during the shaping stage (hizukuri) where the blade is formed, the smith proceeds with forging while consciously targeting a specific sori form and amount.

During the quenching process, the blade is heated in a state where clay (tsuchiwoki) has been applied to its surface, then quenched rapidly in water. At this point, the difference in contraction rates between the blade side (where the clay is thin and cools rapidly) and the spine side (where the clay is thick and cooling is gradual) creates a natural sori in the blade. To achieve the target sori after quenching, the smith intentionally curves the blade before quenching in a slight reverse sori (sakasori)—curving it in the opposite direction to the intended final curve.

The final state of sori results from a complex interplay of multiple factors—the characteristics of the steel, the thickness of the clay application, the heating temperature, the water temperature, and more—and demands the smith's experience and technical judgment. That swords made by renowned masters possess beautiful sori is also a testament to their technical skill in precisely controlling these complex variables.

Changes in Sori and Value Assessment—Transformation Through Polishing and Use

Swords may experience changes in sori over time due to polishing, use, and storage conditions. Repeated polishing thins the blade, and changes in weight distribution can cause subtle shifts in sori. Additionally, changes in humidity and temperature can cause the wooden shirasaya (plain scabbard) to warp, potentially affecting the blade's sori during long-term storage.

In authentication, changes in sori become one of the elements used to evaluate the "period consistency" of a sword. It is not uncommon for tachi made during the Heian and Kamakura periods to have experienced changes in sori due to later polishing, and estimating the original sori form while accounting for such changes is a hallmark of experienced appraisers.

For modern sword collectors, sori is one of the first items to check when verifying that an authentication certificate matches the actual sword. If the sori measurement recorded on a registration certificate or authentication certificate differs significantly from the measured value of the actual sword, questions about the sword's identity may arise, making pre-transaction verification essential.