Why are katanas curved?

by Niklas Feurstein
6 comments
Katana and scabbard

Have you ever wondered why katanas have a curve?

What if I tell you that during the forging process the blade actually is straight.

You don’t believe me? Let’s check out the reasons for katana curvature together.

Reasons for katana curvature

Actually, there is only one reason why katanas are curved.

Just a few minutes ago I read a few forum posts about katana curvature.

It makes me angry that people are sharing incorrect information on this topic all the time.

Exactly this is the reason why I created this blog post. The katana isn’t curved in order to optimize it’s cutting abilities.

The curve on a katana is so slight I consider it to be more similar to a straight blade than a curved blade.

The curvature of a katana is approximately 1.5cm and called Sori in Japanese.

For this reason, katanas don’t cut better than straight swords nor can be drawn quicker from the saya (scabbard).

However, most forums (and other bloggers) claim that this is the case.

I can’t really blame them, as this info I am going to share with you is quite unknown to most people.

Remember, at the start of this post I told you that katanas are still straight blades during the forging process.

Heat Treating a W1 Katana
In this video from Niels Provos, you see the quenching of a katana. The formerly straight blade gets curved.

They get their curve from the tempering process. Almost all katanas are clay tempered.

During this process, the spine of the blade gets covered with clay.

Therefore, the spine remains soft, while the edge gets very hard.

After the heat treatment, the Japanese sword gets quenched.

Due to the clay, the edge cools much faster than the spine. Thus, becoming harder.

The hard edge expands as the metal becomes hardened steel. The spine does not expand so much, curving the formerly straight blade.

Let’s sum this up again in one sentence:

The curvature of a katana comes from the quenching process, as the blade is differentially hardened.

Benefits of katana curvature

I could talk about the forum posts in this section again.

Presumably, you already know what I want to say. 😀

In contrast to the popular opinion, I am certain that the slight curve of the katana has NO effect on its properties.

There is nothing a katana can do better than a medieval sword (at least regarding curvature).

However, let’s address some of the common statements we hear on katana curvature.

Thesis 1: The curve makes cuts more lethal

This is probably the most common misconception about the katana curve.

Curvature might be an advantage if the katana wasn’t so slightly curved. Middle Eastern swords like the heavily curved scimitar certainly got an edge over a straight sword.

Katana vs Scimitar

The curve of the katana is just too slight.

Let me tell you the main reason why people assume that curvature increases the performance of the katana.

This is mostly due to the fact that with a curved blade less of the katana’s edge touches the target.

Thus, the same force is distributed among a smaller surface area. Therefore, the strike will be more harmful.

Most of these tests are done on completely rectangular objects. The curve of the katana decreases the surface that touches the target by around 1cm.

However, no human is anywhere near rectangular. Performing the same test on a more human-like object, resulting in the katana having no advantage over a straight blade.

Whereas the curvature of a scimitar certainly helps build up more force.

Thesis 2: The curve makes drawing the sword easier

In theory, the wielder is able to draw his katana faster, because of its curvature.

It aligns with the drawing motion and makes cutting while drawing possible. The Japanese also came up with a word for this called Nukitsuke.

However, this the curvature of the katana plays no role in this process. This is just a huge myth!

Cutting while drawing is also possible with a straight blade. The factor that influences this point the most is the skill of the samurai.

Nukitsuke was an art. The samurai perfected this technique in order to quickly obliterate of their enemies.

Thesis 3: The curve makes the sword look better

Katana With Saya

Actually, I never heard anyone say this, but it is completely true!

You don’t believe me? Just look at a medieval longsword and compare it to my katana. Which one looks better?

My katana of course. I personally know no one that thinks that a longsword looks better.

Remember it is not about what you LIKE better. Even if you are a knight fan you have to admit that a katana looks prettier.

This is due to the reason that humans possess a subconscious feeling of what looks right and what doesn’t.

Take a look at a single-edged straight sword. Focus on the tip of the blade.

The tip points upwards right? Exactly this is the case (and the problem). The blade is straight, but the point moves upwards.

These two different movements confuse our brain. As a consequence, it doesn’t look as pleasing for the eyes.

Now let’s move on to the katana. Same game. The tip points upwards. However, because of the curvature the blade moves in the direction the kissaki (point) is facing.

Due to this we subconsciously think that the katana looks better than a regular straight blade.

I haven’t had the time to interview many more people on this topic. I would really appreciate sharing your opinion in the comments. (or disagree).

Conclusion

Yeah, I always like to give a quick summary at the end of my posts.

In comparison to Middle Eastern swords, the katana only has a very slight curve. The curvature of a katana is more similar to a medieval sword than to a scimitar for example.

The curvature of the katana is too slight to be advantageous. However, no one can deny that the curvature is a big enhancement of the katana’s aesthetics.

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6 comments

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William Z 10. November 2020 - 3:48

Hello,
Sorry for sharing in this thread I didn’t start, but in my honest opinion, the evolvement of the Japanese sword in history pivots around two central themes: the durability of the sword, and its ability to cut (slice is a more proper term).
The Japanese sword was mainly inspired by its Continental counterparts, the Chinese and the Korean swords; Japanese swords were straight, mostly single-edged swords, “chokuto” blades. Near the middle of the Heian period, warfare was waged mostly on horseback, which required a change in the shape and length of swords: the tachi was born. Afterwards, either by a happy accident or the application of genius, the swordsmithing technique of using variable hardness throughout the blade and using differential hardening when tempering was born. This also contributed in making extremely sharp but resistant swords, which helped to create the legendary status of the later katana.

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Niklas Feurstein
Niklas Feurstein 10. November 2020 - 9:25

It is always great to get more and more different thoughts and reasons for this topic.

Yes, the tachi is indeed a horseback weapon with a more curved blade. I wrote an article about this specific topic as well.

So in your opinion, the reason why the katana is slightly curved is that it is an evolution from the tachi and primarily a side effect of the hardening process?

While both may be true that does not completely explain the slight curvature. A swordsmith could have given it a more pronounced curve, or an expert could even differentially harden it in such a way to retain a more or less straight shape.

Hm. Maybe they tried to make it as straight as possible with little effort. That certainly is worth thinking about. Thanks for your input.

Niklas Feurstein

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Ash 24. April 2020 - 17:45

I can maybe agree that it doesn’t increase cutting capacity. Maybe. But it is fact that it both draws faster and makes Nukitsuke easier, although that is more commonly called iaijutsu. If it were not more effective, then why would there be an entire martial art, with multiple styles, dedicated to just drawing the sword and putting it back? Look up iaido. The single-edged, curved shape allows you to pull the back of the blade against the saya, allowing you to draw quicker and increasing the force of the draw strike. If you tried to do the same with most straight swords, as they tend to be double-edged, it would not go so well for your hands.

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Niklas Feurstein 30. April 2020 - 11:10

You are completely right that Nukitsuke won’t work on a double-edged sword. However, what I wanted to say is that the curvature of the katana is to slight to make a difference in this art.

In my opinion a straight single-edged blade will perform nearly identical. In order to benefit more the katana would have to be as curved as a scimitar. (As this increases the force you can build up)

The tachi for example has a more curved blade. Strangely it was worn with the edge facing downwards (at least according to historians) which means that it wasn’t mainly used for cutting while drawing.

Currently I don’t own a tachi so I can’t really test the difference if both are worn edge facing upwards …

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zazuge 21. October 2020 - 13:16

the cut and draw based martial art “iaido”
had mainly evolved in the shogunate reign during the time of peace
means it had nothing to do with war itself, the katana certainly evolved to be use in streets and indoors, because after the end of war, the samurai class had nothing to do, but they continued to fight for individual reasons and after losing their purpose, they were trying desperately to cling to honor, so any slight of hand or dispute ended in a duel or a street fight
it’s an entirely different context from other places where wars shaped the swords and the fighting styles

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Niklas Feurstein
Niklas Feurstein 21. October 2020 - 14:21

Hey.

it’s an entirely different context from other places where wars shaped the swords and the fighting styles

Yes, that may be a reason why it is curved and looks better than European swords. But I don’t completely agree with you on that.
Even though the katana was created in the Kamakura period it became popular and widely used later on in the Muromachi period. Which is followed by the Sengoku period that had a lot of military conflicts. So in my opinion it was influenced by wars (and nonetheless it was created to be used in wars).

Regarding you statement with iaido, it may be the case that nukitsuke was a way to show off your style and honor. Iaido surely was a good way to stay in shape and master the sword.

Yours,
Niklas

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