5 Types of Sori (Curve) Found on Japanese Blades

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.

What Exactly Is a Sori?

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 mainly seen on sword blades:

1. Koshizori (Curvature near the Tang)

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.

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)

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).

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)

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.

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.

Sori mainly seen on tanto blades:

4. Uchizori (Inward Curvature)

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.

5. Takenoko-zori (Bamboo-Shoot Curvature)

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.

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.