What is Oosujikai in Japanese Katana terminology?

The term "Oosujikai" (大筋違, おおすじかい) refers to a specific pattern of file marks on a Japanese sword. In this pattern, the angle of the file marks is significantly lowered from the blade side towards the spine side. This creates a distinct visual effect on the surface of the blade, contributing to its overall aesthetic appeal. The term "Oosujikai" is derived from the Japanese words "Oo" (large), "Suji" (line or streak), and "Kai" (change or difference), reflecting the prominent and dramatic change in the angle of the file marks in this pattern. This is one of many different file mark patterns that can be found on Japanese swords, each with its own unique characteristics and historical significance.