What is Koshizori in Japanese Katana terminology?

"""Koshizori"" (腰反り) refers to a curvature style often seen in Tachi (long swords) from the late Heian period to the early Muromachi period. In this style, the most pronounced curve is near the Machi (notch), which is the area where the blade meets the tang. This style is particularly common in Bizen swords from the Koto period.

In the late Edo period, swordsmiths who used old Bizen works as models also produced similar works. This curvature style is not only aesthetically pleasing but also functional, as it affects the sword's balance and cutting ability. The continued use of Koshizori in different periods reflects its effectiveness and the enduring influence of Bizen sword-making techniques."