The tachi (太刀) is a Japanese sword, often said to be more curved and slightly longer than the katana. The daito (long swords) that pre-date the katana average about 78cm in blade length, next to the katana's average of around 70cm. As opposed to the traditional manner of wearing the katana, the tachi was worn hung from the belt with the cutting-edge down, and usually used by cavalry. Deviations from the average length of tachi have the prefixes ko- for "short" and o- for "great" attached. For instance, tachi that were shoto and closer in size to a wakizashi were called "kodachi". The longest tachi (considered a 15th century "odachi") in existence is more than 3.7 meters in total length (2.2m blade) but believed to be ceremonial.

It was the predecessor to the katana as the battle-blade of feudal Japan's bushi warrior class, and as it evolved into the later design, the two were often differentiated from each other only by how they were worn and by the fittings for the blades. In later Japanese feudal history, during the Sengoku and Edo periods, certain high-ranking warriors of what became the ruling class would wear their sword tachi-style (edge-downward), rather than with the saya (scabbard) thrust through the obi (belt) with the edge upward.

See also

• tsurugi
• wakizashi
• kodachi
• katana
• nodachi
• odachi
• daito