What is Sansaku Boshi in Japanese Katana terminology?

The term ""三作帽子"" (Sansaku Boshi) refers to a type of tempering pattern on a Japanese sword where the pattern becomes loose at the Yokote line, droops towards the tip, and returns in a small circle. This pattern is said to be a common feature of the works of the swordsmiths ""Nagamitsu"", ""Sanenaga"", and ""Kagemitsu"", who were active in Bizen Province (now eastern Okayama Prefecture).

""Boushi"" refers to the pattern of the blade that appears on the tip/cutting edge of a sword. It is speculated that the term originates from the word for a hat that is worn on the head. When written, it may also be written as ""鋩子"", but there is no significant meaning in the difference in kanji.

The ""Boushi"" is an important part in the study of swords, and not only does it reveal the personality and characteristics of the swordsmith, but it is also a part where the era and lineage of the sword making cannot be hidden. Therefore, it is sometimes likened to the ""face"" of a human.

In addition, in sword shops, it is said that ""do not let your guard down with customers who look closely at the 'Boushi' and the 'Yakidashi'"". In the sword appraisal house ""Honami family"", there is a rule that says ""When judging a sword without a signature, the final decision is made by the 'Boushi'. No matter how well the other parts are made, the appraisal result is determined by the 'Boushi'.