This chapter is a continuation of Chapter 3, Names of Parts. Please read Chapter 3, Names of Parts, before reading this section.
This chapter explains how to identify the koshi-zori or chukan-zori. Chukan-zori is also called torii-zori or kyo-zori. Chukan-zori means that the most curved part of the sword’s body is around the middle, and for koshi-zori, the most curved part is located lower than the center of the blade, approximately 1/3 down the body. Every sword seems to have its curvature around the middle area, especially when looking at photos of swords in books. This is because these swords are positioned to fit nicely within a given rectangular photo space.
The correct way to check for the curvature is to hold the nakago (茎) vertically. This way, you can see the location of the curvature more accurately. If the nakago is not vertical, the curvature appears in the middle section of every sword. When examining a sword, the first step is to hold the sword and make sure the nakago stands vertically. When looking at a sword in a book, rotate (shift or slide) the book slightly so that the nakago is perpendicular. This way, you can see the precise location of the curvature. Keep in mind that the difference can sometimes be subtle.
Study of Japanese Sword 