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.

In swordsmithing, hamon (刃文) is a visible effect created on the blade by the hardening process. The hamon is the outline of the hardened zone which contains the cutting edge. Blades made in this manner are known as differentially hardened, with a harder cutting edge than spine. This difference in hardness results from clay being applied on the blade prior to the cooling process (quenching). Less or no clay allows the edge to cool faster, making it harder but more brittle, while more clay allows the center and spine to cool slower, thus retaining its resilience.