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5 Types of Sori (Curve) Found on Japanese Blades

Written By: Abigail Cambal
Published On: July 5, 2024

Sori, the curvature of a Japanese blade, can range from shallow to deep, with the most pronounced part of the curve varying along the entire length of the blade. There are different types of sori, each associated with specific periods of sword forging, making the curvature a good indicator of a sword’s production time.

KEY TAKEAWAYS
Sori refers to the curvature of a Japanese sword or dagger blade.
The curvature of the blade is described in terms of its location, namely: koshizori, toriizori, sakizori, uchizori, and takenoko-zori.
The most common type of sori found on Japanese swords is the toriizori, in which the deepest part of the curvature is located at the center of the blade.

What Exactly Is a Sori?

A Japanese blade showing how a curvature or sori is measured
A Japanese blade showing how a curvature or sori is measured – Credits: Samurai Museum

The sori (反り) refers to the curvature of a Japanese sword or dagger. It is measured between the deepest curve of the back surface (mune) and an imaginary straight line drawn from the notch (munemachi) at the top of the tang to the tip (kissaki).

If the blade has a curvature, the sori is the longest horizontal distance between the back surface (mune) and the imaginary straight line. This is also where the curvature of the blade is at its maximum. The depth of curvature can vary, described as either shallow or deep.

The sori is also characterized by its location on the blade, which can be near the tang, in the center, or near the tip. While several blades may share the same amount of curvature, their sori locations may vary, and they may feature differently shaped tangs (nakago).

Different Types of Sori (Curvature)

The sori of Japanese swords sets them apart from early chokuto, or straight swords. The depth and location of the curvature vary depending on the period in which the blade was forged, as Japanese swords underwent dramatic changes to suit the military tactics of the time.

Sori is typically pronounced and spelled as zori when preceded by other characters, hence koshizori, toriizori, sakizori, and so on. The term muzori (無反り), meaning no sori, is used to describe a tanto blade without curvature.

Sori mainly seen on sword blades:

Sword blades showing various types of curvature
(From top to bottom) Sword blades showing various types of curvature: koshizori, toriizori, sakizori, respectively – Credits: Markus Sesko

1. Koshizori (Curvature near the Tang)

Japanese blades with koshizori
A Japanese long sword with koshizori, in which the deepest part of the curvature is near the tang (top), and a slim Japanese blade with a deep koshizori – Credits: e-Museum

The term koshi means waist. The koshizori (腰反り) refers to a curvature with its deepest curve located towards the hilt of the sword, particularly noticeable on the munemachi—the notch at the base of the back surface (mune) of the blade, near the top of the tang.

A pair of daisho with koshizori
A pair of daisho, with the longer sword above featuring a slightly shortened katana blade with a koshizori – Credits: Mandarin Mansion Antiques

The koshizori is sometimes referred to as the Bizen-zori (備前反り), as swords from Bizen province typically have this curvature. Also, the tachi swords produced during the Heian and mid-Kamakura periods commonly feature the koshizori.

2. Toriizori (Curvature at the Center)

A katana featuring a toriizori
A katana featuring a toriizori, with the curvature deepest at the center – Credits: Mandarin Mansion Antiques

The term torii refers to the Shinto shrine archway, after which this type of curvature is named. Toriizori (鳥居反り) describes a curvature with its deepest curve located around the center of the blade. It resembles the curved crossbar (kasagi) of the torii archway and is sometimes referred to as kasagi-zori (笠木反り).

A katana with a moderate and even toriizori
A katana with a moderate and even toriizori – Credits: Mandarin Mansion Antiques

The toriizori is the most common type of curvature found on Japanese swords. It is occasionally known as Kyō-zori (京反り), as the Yamashiro school, also referred to as Kyo, commonly applied this curvature on their blades.

3. Sakizori (Curvature near the Tip)

An oshigata of a wakizashi with a relatively prominent sakizori
An oshigata of a wakizashi with a relatively prominent sakizori – Credits: Markus Sesko

The term saki means upper. Sakizori (先反り) describes a curvature where the deepest curve is located in the upper portion of the blade. This type of curvature is often observed on swords produced during the Muromachi period.

A tanto blade featuring a sakizori
A tanto blade featuring a sakizori – Credits: Nihonto craft

The sakizori on tanto daggers did not become prevalent until the Muromachi period and is primarily observed on blades that are oversized tanto, such as sunnobi-tanto, or hira-zukuri ko-wakizashi.

Blades on Japanese naginata polearms typically feature a pronounced sakizori. This characteristic is mainly found in naginata from the Shinto era. However, some naginata, such as those crafted by Masatsune , may resemble the traditional shapes of the Kamakura period.

Sori mainly seen on tanto blades:

Dagger blades showing uchizori slight sori and takenokozori
(From top to bottom) Dagger blades showing uchizori, slight sori, and takenokozori, respectively – Credits: Markus Sesko

4. Uchizori (Inward Curvature)

A tanto blade with a slight uchizori
A tanto blade with a slight uchizori – Credits: Bonhams
A sunnobi tanto blade with uchizori
A sunnobi-tanto blade with uchizori – Credits: Markus Sesko

The term uchi means inward or inside. Uchizori (内反り) refers to a curvature where the back of the blade curves towards the cutting edge. However, blades were not originally made by swordsmiths to have uchizori. Instead, it typically results from blade repair, overpolishing, or alterations. Tanto produced during the Kamakura period commonly exhibit uchizori.

Some chokuto (straight swords) produced during the Nara period feature a thick blade with uchizori, characterized by a slight curvature of the back surface curving toward the cutting edge.

5. Takenoko-zori (Bamboo-Shoot Curvature)

A tanto blade featuring a takenoko zori
A tanto blade featuring a takenoko-zori, or bamboo-shoot curvature – Credits: Aoi Japan

The takenoko-zori (筍反り) refers to a curvature seen on certain tanto blades, resembling the shape of bamboo shoots (takenoko). It is typically seen on tanto blades with uchizori and scarce fukura—the bulbousness of the cutting edge at the tip (kissaki).

The takenoko-zori appeared on some tanto from the late Kamakura period and is notable in blades crafted by Norishige (則重). In these blades, the combination of uchizori and scarce fukura results in a tapered shape towards the tip.

Examining the Sori in Sword Appraisal

In Japanese sword appraisal, experts determine the provenance of a blade, as well as its swordsmith or swordmaking school, based on its appearance. When examining the shape (sugata) of the blade, the sori or the curvature is among the factors examined. The curvature of a blade may reveal its production time, as Japanese swords changed over time to adapt to fighting conditions.

Experts examine the curvature by holding the blade vertically. Alternatively, the blade can be held horizontally, with the butt end of the tang near eye level. When examining the back surface (mune) in this position, it becomes easier to observe how the blade curves.

Sometimes, the differences in curvature are subtle, making it uncommon to encounter a blade with a prominent sakizori  or koshizori (curve near the tang).

In Sword Blades

According to Mr. Markus Sesko, an expert in Japanese arms and armor, a clearly visible koshizori (curve near the tang) is more likely in the Kamakura period. On the other hand, a prominent sakizori (curve near the point area) is more likely in the Muromachi period, while a subtle sakizori may point to the end of the Nanbokucho or early Muromachi periods.

In Dagger Blades

A tanto dagger blade may appear short and straight, but experts examine whether it really lacks curvature (muzori) or shows an uchizori shape, where the back surface curves downward toward the edge. It is important to note that tanto were never originally forged in an uchizori shape. Instead, years of polishing and wear on a straight tanto can give the impression of an uchizori blade.

A tanto blade with an uchizori (inward curvature) suggests Kamakura period or a later production paying homage to the Kamakura style. A takenoko-zori (bamboo-shoot curvature) indicates the late Kamakura period, and seen in the works of swordsmith Norishige (則重).

However, a sakizori (curve near the point area) is only found in large variations of dagger referred to as sunnobi-tanto or hira-zukuri ko-wakizashi. The more prominent sakizori, the later its production time in the Muromachi period.

Sources Cited

  1. Kapp, L., Kapp, H., & Yoshihara, Y. (2013). Modern Japanese Swords and Swordsmiths: From 1868 to the Present. Kodansha USA.
  2. Katakiriha zukuri naginata by Masatsune. (n.d.). Mandarin Mansion. Retrieved March 24, 2024, from https://www.mandarinmansion.com/item/katakiriha-zukuri-naginata-masatsune
  3. Nagayama, K. (2017). The Connoisseur’s Book of Japanese Swords. Kodansha USA.
  4. Roach, C. M. (2010). Japanese Swords: Cultural Icons of a Nation. Tuttle Publishing.
  5. Sesko, M. (2014). Encyclopedia of Japanese Swords. Lulu.com.
  6. Sesko, M. (2015, February 14). KANTEI 1 – SUGATA #1. Markus Sesko. Retrieved March 24, 2024, from https://markussesko.com/2015/02/14/kantei-1-sugata-1/
  7. Tsuchiko, T. (2002). 日本刀21世紀への挑戦: The New Generation of Japanese Swordsmiths 英文版 (K. Mishina, Trans.). Kodansha International.
  8. Yoshihara, Y. (2012). The Art of the Japanese Sword: The Craft of Swordmaking and its Appreciation. Tuttle Publishing.

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