A tachi (太刀?) was a type of Japanese sword worn by the samurai class of feudal Japan. The tachi style of swords preceded the development of the katana — the first use of the word katana to indicate a blade different from tachi appears toward the end of the twelfth century.
The production of swords in Japan is divided into specific time periods; Jōkotō (Ancient swords, until around 900 A.D.), kotō (old swords from around 900-1596), shintō (new swords 1596-1780), shinshintō (new new swords 1781-1876), gendaitō (modern swords 1876-1945). and shinsakutō (newly made swords 1953–present).
Authentic tachi were forged during the Kotō period, before 1596. With a few exceptions katana and tachi can be distinguished from each other if signed, by the location of the signature (mei) on the tang (nakago). In general themei should be carved into the side of the nakago that would face outward when the sword was worn. Since a tachiwas worn cutting edge down, and the katana was worn cutting edge up the mei would be in opposite locations on the nakago of both types of swords.
An authentic tachi that was manufactured in the correct time period averaged 70–80 centimeters (27 9/16 – 31 1/2 inches)in cutting edge length (nagasa) and compared to a katana was generally lighter in weight in proportion to its length, had a greater taper from hilt to point, was more curved with a smaller point area.
Unlike the traditional manner of wearing the katana, the tachi was worn hung from the belt with the cutting-edge down, and was most effective when 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 late 1500s and early 1600s many old surviving tachi blades were converted into katana by having their original tangs cut (o-suriage), the signature (mei) would be lost in this process.
For a sword to be worn in “tachi style” it needed to be mounted in a tachi koshirae. The tachi koshirae had two hangers (ashi) which allowed the tachi to be worn in a horizontal position with the cutting edge down. A sword not mounted in a tachi koshirae could be worn tachi style by use of a koshiate, a leather device which would allow any sword to be worn in the tachi style.
According to author Karl F. Friday, before the 13th century there are no written references or drawings etc that show swords of any kind were actually used while on horseback.
The uchigatana was derived from the tachi and 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 tachi and the uchigatana were often differentiated from each other only by how they were worn, the fittings for the blades, and the location of the signature (mei).
As a result of the first Mongol invasion (1274) tachi started to be made thicker and wider.
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 swords called Shin guntō and Kai guntō which were worn tachi style (cutting edge down).