The tachi is a Japanese sword, often said to be more curved and slightly longer than the katana. However, it has been stated that a sword is called a tachi when hung from the obi (belt or sash) with the edge down, and the same sword becomes a katana when worn edge up and thrust through the girdle.
The tachi was used primarily on horseback, where it was able to be drawn efficiently for cutting down enemy footsoldiers. On the ground it was still an effective weapon, but somewhat awkward to use.
It was during the Mongol invasions that it was shown there were some weaknesses in the tachi sword which led to the development of the Katana.
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 belt with the edge upward. However, the tachi style was eventually discarded in favor of the katana. In the year 1600, many old tachi were actually cut down into katana. The majority of surviving tachi blades now are o-suriage, so it is rare to see an original signed ubu tachi.
The longest tachi (considered a 15th century odachi) in existence is more than 3.7 meters in total length (2.2m blade) but it is believed to have been ceremonial.