Gorō Nyūdō Masamune was a medieval Japanese blacksmith widely acclaimed as Japan's greatest swordsmith. He created swords and daggers, known in Japanese as tachi and tantō, in the Sōshū school. However, many of his forged tachi were made into katana by cutting the tang (nakago) in later times ("suriage"). For this reason, his only existing works are katana, tantō, and wakizashi. No exact dates are known for Masamune's life. It is generally agreed that he made most of his swords between 1288 and 1328. Some stories list his family name as Okazaki, but some experts believe this is a fabrication to enhance the standing of the Tokugawa family.

A tachi is a type of sabre-like traditionally made Japanese sword (nihonto) worn by the samurai class of feudal Japan. Tachi and uchigatana generally differ in length, degree of curvature, and how they were worn when sheathed, the latter depending on the location of the mei (銘), or signature, on the tang. The tachi style of swords preceded the development of the katana, which was not mentioned by name until near the end of the twelfth century. Tachi were the mainstream Japanese swords of the Kotō period between 900 and 1596. Even after the Muromachi period (1336–1573), when katana became the mainstream, tachi were often worn by high-ranking samurai.