The tachi (太刀, Hiragana: たち) is a Japanese sword, often said to be more curved and slightly longer than the katana. However, Gilbertson, Oscar Ratti, and Adele Westbrook state 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" style was eventually discarded in favor of the katana.
The daitō (long swords) that pre-date the katana average about 78 cm in blade length, larger than the katana average of around 70 cm. Unlike the traditional manner of wearing the katana, the tachi was worn hung from the belt with the cutting-edge down, and was usually used by cavalry. Deviations from the average length of tachi have the prefixes ko- for "short" and ō- for "great or large" attached. For instance, tachi that were shōtō and closer in size to a wakizashi were called kodachi. The longest tachi (considered a 15th century ōdachi) in existence is more than 3.7 meters in total length (2.2m blade) but believed to be ceremonial. In the year 1600, many old tachi were cut down into katana. The majority of surviving tachi blades now are o-suriage (having their original tangs cut at some point), so it is rare to see an original signed ubu (original tang complete and unaltered) tachi.
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. This is why its companion, the uchigatana (the predecessor of the katana) was developed.
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.
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.
With the rising of militarism during the Shōwa era, the Imperial Japanese Army and the Imperial Japanese Navy implemented tachi called Shin guntō and Kai guntō.
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