What is Sensuki in Japanese Katana terminology?

The term "Sensuki" (せんすき, せんすき) refers to a specific pattern of file marks (Yasurime) on a Japanese sword, created using a file called "Sensuki". These file marks are applied to the surface of the tang (Nakago) of the sword to prevent it from slipping out of the handle. From the Shinto period onwards, decorative file marks known as "Kesho Yasuri" became common. However, in the Koto period and in the construction of spears and Naginata, a simple pattern resembling file marks was created by gouging out the surface with a hook-like tool. This terminology is used to describe the specific characteristics of the sword's structure, contributing to its overall aesthetic and functional design.