to the kami of Shinto shrines and fell out of use as weapons. On the other hand, in the Kamakura period, there was a type of tachi called hirumaki tachi (蛭巻太刀) with a scabbard covered with metal, which was used as a weapon until the Muromachi period. The meaning was a sword wrapped around a leech, and its feature was that a thin metal plate was spirally wrapped around the scabbard, so it was both sturdy and decorative, and chains were not used to hang the scabbard around the waist. The Mongol invasions of Japan in the 13th century during the Kamakura period facilitated a change in the designs of Japanese swords. The swordsmiths of the Sōshū school represented by Masamune studied ruined tachi -- broken or bent in battle -- to develop new production methods, and create innovative Japanese swords. They forged the blade using a combination of soft and hard steel to optimize the temperature and timing of the heating and cooling of the blade, resulting in a lighter and very robust blade. They also made the curve of the blade gentle, lengthened the tip linearly, widened the width from the cutting edge to the opposite side of the blade, and thinned the cross section to improve the penetration and cutting ability of the blade. Historically in Japan, the ideal blade of a Japanese sword is considered to be the kotō in the Kamakura period, and the swordsmiths from the Edo period to the present day after the Shintō period focused on reproducing the blade of a Japanese sword in the Kamakura period. There are more than 100 Japanese swords designated as National Treasures in Japan, of which the Kotō of the Kamakura period account for 80% and the tachi account for 70%. From the end of the Kamakura period to the end of the Muromachi period (1333–1573), kawatsutsumi tachi (革包太刀), which means a tachi wrapped in leather, was popular. The kawatsutsumi tachi was stronger than the kurourushi tachi because its hilt was wrapped in leather or ray skin, lacquer was painted on top of it, leather straps and cords were wrapped around it, and the scabbard and sometimes the tsuba (hand guard) were also wrapped in leather. By the 15th century, Japanese swords, including tachi, had already gained international fame by being exported to China and Korea. For example, Korea learned how to make Japanese swords by