What is Katasujikai in Japanese Katana terminology?

The term "Katasujikai" (片筋違, かたすじかい) refers to a specific pattern of file marks on a Japanese sword. In this pattern, the file marks start off as vertical to the blade, known as "Kiri" (切り), and then transition into a diagonal pattern, known as "Sujikai" (筋違, すじかい). This creates a unique visual effect on the surface of the blade, contributing to its overall aesthetic appeal. The term "Katasujikai" is derived from the Japanese words "Kata" (one side), "Suji" (line or streak), and "Kai" (change or difference), reflecting the distinctive 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.