腰 (Koshi)

What is Koshi in Japanese Katana terminology?

Koshi refers to the slope between the 'head' (Yaki no Kashira) and the 'valley' (Yaki no Tani) in the blade pattern (Hamon) of a Japanese sword. In blade pattern (Hamon)s such as "互の目" (Gunome), the slope is often described as "腰が開く" (Koshi ga Hiraku), which means the slope is wide and gentle. This term is used to describe the curvature of the blade pattern (Hamon), which can greatly affect the overall appearance and aesthetic of the sword.