Introducing The Sori

The distinctive curve of Japanese swords symbolizes their history. It not only enhances a swordsman's appearance but also enables him to master more extraordinary skills. This curve is known as Sori by swordsmiths. So, what's a Sori, exactly? How did it become so significant?
A Samurai sword blade's curvature is known as Sori. Such curves help with your "hiki-nuke" movement during a fight. An ideal Sori thickness varies from 0.5 to 1 inch. However, it can go up to 1.5 inches at max. Five types of Sori define the differences between katanas. The types are Mu Sori, Saki Sori, Uchi Sori, Koshi Sori, and Tori Sori.
Swordsmiths fuse carbon with metal to make durable and robust katanas. It takes days of dedication and hard work to forge a single katana.

What Is Sori?

Sori is known as the curvature of a Samurai sword. Sometimes the samurai or other swordsmen call it "Zori." Sori is related to a Katana's back part, also called Mune.
Basically, Sori determines the durability and strength of a Katana. You can quickly tell or measure the Sori of a Katana with a simple trick. At first, you need to imagine a straight line between Munemachi and kissaki. It means from the part near the handle to the tip of the Katana.
And then detect the deepest point of your Katana's curvature. After that, imagine another straight line that will intersect the first one. And that's the point of measurement. Typically, the maximum thickness varies between 1.25 inches and 1.5 inches. But the ideal thickness is about 0.5 to 1 inch.

Why Curved?

The reason behind the curvature of Katana is the "hiki-nuke" movement. It's like striking down your sword and immediately pulling it backward. Your arm needs to move in a circular trajectory. And your shoulder should be in the center.
This movement is pretty effective and quick. So, the curvature, or Sori, helps you move or strike with your katana more effectively than a straight sword.

A Little Bit Of History

Contrary to popular belief, Japanese Samurai Swords don't have one specific design. The sword types vary based on curvature.
The Heian Era gave birth to Samurai swords in history. However, the first ones were straight with double edges. The blades became less tapered and wider during the Kamakura period. And the blades became legendary during the Nanbokucho era because of their curves. But they were too heavy.
However, the Katanas were born in the Muromachi era. That's because their deepest curves have compatible lengths. The classic katana from the Muromachi era can still be found today.
Lastly, the Edo era brings variations in blade sizes and shapes. Katanas were forged in sizes ranging from 26 to 29 inches.

Types Of Sori

As mentioned, Sori is basically the curvature of a Japanese sword or katana. However, it varies from sword to sword. If you are a Katana user, you will immediately know the differences. But for newbies, it's hard to classify. Here are the types of Sori:

Tori Sori (Kyo Sori)

Tori refers to the gateway to a Shinto shrine, and Kyo derives from the Yamashiro province. The Tori Sori resembles the curved crosspiece of the Katana.
Tori Sori is located at the deepest and middle points of a Katana. It's also at the perfect halfway point of your sword. Therefore, you can easily differentiate Tori Sori from Koshi Sori.

Koshi Sori (Bizen Sori)

The word "koshi" means waist. So, the Koshi Sori is at the deepest point of the curvature, near the waist of your katana.
In another way, Koshi Sori is located between the Munemachi and the blade. Therefore, it's closer to tang compared to the kissaki. Some swordsmen also call it Bizen Sori because the name has been in use since the Kamakura period.

Uchi Sori (Takenoko Sori)

Uchi means inner. It means the curve is on the inward part of your Katana. The best example of such Uchi Sori is the Tanto blade.
That's why swordsmen also call the Uchi Sori "takenoko." Here, takenoko means bamboo shoot.

Saki Sori

The name Saki Sori comes from the word Saki which refers to "upper." That's because the Sori is located at the halfway point of the Katana, towards the Kissaki or tip.
Saki Sori was pretty common during the Muromachi period. You will notice Saki Sori in most Naginata blades.

Mu Sori (Chukan Sori)

The Mu Sori is a bit different from other Sori. That's because Mu Sori refers to the straight form.
Japanese people use the Mu word to refer to Tanto blades without any curve. People call them straight swords from the archaic period. Chukan Sori's deepest curve is right in the middle of the Katana.

Swordsmith's Dedication And Passion Towards A Sori

Sword curves help with cutting more than thrusting. So, the swordsmiths make the katanas sharper to cut better with greater strength.
Due to the advancement of steel metallurgy, swordsmiths forsake bronze and use particular types of steel to make Katanas. The steel is also known as Tamahagane. It won't break or be damaged when the swordsmith tries to make a curve.
Japanese sword blades get their curvature through quenching (clay tempering), not forging. Smith uses multiple layers of steel sheet by varying the carbon levels. The layers must be distributed evenly to eliminate impurities. After several stretches and folds, it forms into a billet. Then the smith uses heat to fuse steel with carbon.
They make the cutting edge thinner than the rest of the katana's body. The process is repeated until the desired curvature or proper curvature is obtained. Each katana has the dedication and passion of several swordsmiths.

Bottom Line

Curvature is the symbol of Japanese katanas. They are also known as Sori. Different types of curves are forged in the middle of a katana to make attacking movement more substantial and more effective.
Hopefully, the discussion on Sori covered all the topics you wanted. Learning about all the types of Sori will help you choose your favorite katana. Most sword owners prefer Tori Sori or Koshi Sori because of their beautiful curvature. Know your Katanas!