What is Tarumi Boushi in Japanese Katana terminology?

The term ""弛み帽子"" (Tarumi Boushi) refers to a type of tempering pattern on a Japanese sword where the pattern appears to sag slightly from the Yokote line towards the tip. The name ""Tarumi Boushi"" comes from the fact that the shape of the tempered edge on the tip (kissaki) does not bulge around the middle, but rather slightly indents inward. This pattern is notably seen in the works of ""Nagamitsu"" from Bizen Province and ""Tanba no Kami Yoshimichi"", and is categorized as ""Sansaku Boushi"" and ""Sanpin Boushi"".

The Yokote line is a line that separates the body of the blade from the tip, and the pattern of the blade that appears on the tip/cutting edge of a sword is called ""Boushi"". The ""Tarumi Boushi"" is a unique and artistic expression of the swordsmith's skill, and it is highly valued by sword enthusiasts and collectors.