The line seen between the edge and spine of a blade where the metal is changes hardness. This is typical in Japanese styled blades from the process of differential tempering or hardening.
the artistic pattern of heat-treated steel
The "line" or visual characteristic typical of Japanese blades caused by the use of differential hardening.
In swordsmithing, (from Japanese, literally "blade pattern") is a visual effect created on the blade by the tempering process. It is the transition between the region of harder martensitic steel of the blade edge and the softer pearlitic steel of the back of the sword. This visual effect is from the harder Martensite crystals in the steel of the edge which make the steel harder.
