The tachi (太刀:たち?) is a Japanese sword that is often described as slightly more curved and longer than the katana.
According to Gilbertson, Oscar Ratti, and Adele Westbrook, a sword is called tachi when it hangs from the obi (belt) with the cutting edge down, and the same sword becomes a katana when worn with the cutting edge up, passing through the waist.
The hi style was eventually discarded in favor of the katana.
The earlier daitō (long swords) before the katana had an average blade length of about 78 cm, longer than the average katana blade length of about 70 cm.
Unlike the traditional way of wearing the katana, the tachi hung from the belt with the cutting edge facing down and was typically used by cavalry.
Deviation from the usual length of the tachi is indicated by adding the prefix ko- (meaning "short") and ō- (meaning "large"). For example, tachi swords that were shorter and closer in size to the wakizashi were called kodachi.
The longest tachi (considered an ōdachi from the 15th century) in existence has a total length of over 3.7 meters (2.2 meters of blade), but it is believed to be ceremonial.
During the year 1600, many old tachi were shortened to the size of a katana.
Most surviving tachi blades today are o-suriage, so it is rare to see an original sword signed as ubu tachi.
It was the first long sword forged in Japan.
It is the direct predecessor of the katana (with greater length).
It continued to be used for many years as a ceremonial sword by samurai of the highest rank.
It was primarily used as a cavalry sword and is designed for cutting from bottom to top.
Usage
The tachi was primarily used on horseback, where it could be efficiently wielded to cut down enemy infantry. However, it remained an effective weapon on the ground, although its use was more complicated compared to when used on horseback. It was for this reason that its counterpart, the uchigatana (predecessor of the katana), was developed.
The tachi was the predecessor of the katana as the battlefield blade of the feudal Japanese bushi (warrior class), and as the design evolved, both weapons were often distinguished from each other mainly by how they were worn and the details of their blades.
In later feudal Japanese history (during the Sengoku and Edo periods), some high-ranking warriors of the emerging ruling class wore their swords in the tachi style (with the cutting edge down), rather than wearing their swords with the scabbard thrust through the belt with the cutting edge up.