Nijūba (二重刃) is a term used in the context of Japanese swords. It refers to a feature where the blade pattern (Hamōn) appears double due to the ""Yubashiri"" or ""Niesuji"" that appear along the blade edge.

The term ""Nijūba"" translates to ""double-edged"" in English. It is named so because the line or dot of ""Nie"" (the visible particles of hardened steel) or ""Nioi"" (the whitish misty appearance along the cutting edge) that runs along the blade pattern, makes it appear as if the blade has been tempered twice.

This feature is often seen in straight tempered blades (Suguha). There are two types of ""Nijūba"": one where the ""Nie"" or ""Nioi"" flows in a line within the blade, making it appear as if there is another blade pattern, and another where the ""Nie"" or ""Nioi"" forms a step-like pattern above the blade edge, making it appear as if the blade pattern is double.

Therefore, ""Nijūba"" adds a unique and interesting visual effect to the blade, enhancing its overall aesthetics and value.