A tachi (太刀) is a type of sabre-like traditionally made Japanese sword (nihonto) worn by the samurai class of feudal Japan.

Tachi and uchigatana generally differ in length, degree of curvature, and how they were worn when sheathed, the latter depending on the location of the mei (銘), or signature, on the tang.

[5][6][7] In the middle of the Heian period (794–1185), samurai improved on the warabitetō to develop kenukigata-tachi [ja] (early Japanese sword).

The term kenukigata is derived from the fact the central part of tang is hollowed in the shape of ancient Japanese tweezers (kenuki).

[9] In the tachi developed after kenukigata-tachi, a structure in which the hilt is fixed to the tang (nakago) with a pin called mekugi was adopted.

[11][full citation needed] However, According to Yoshikazu Kondo, bow and arrows were certainly the main weapons used in cavalry battles, but from around the Genpei War in the 12th century, the use of tachi on horseback increased.

On the other hand, court nobles wore tachi decorated with precisely carved metal and jewels for ceremonial purposes.

The kazatachi and hosodachi worn by nobles were initially straight like a chokutō, but since the Kamakura period they have had a gentle curve under the influence of tachi.

At the end of the Kamakura period, simplified hyogo gusari tachi came to be made as an offering to the kami of Shinto shrines and fell out of use as weapons.

The swordsmiths of the Sōshū school represented by Masamune studied ruined tachi – broken or bent in battle – to develop new production methods, and create innovative swords.

Furthermore, in the late 16th century, Tanegashima (matchlock arquebuses) were introduced from Portugal, and Japanese swordsmiths mass-produced improved products, with ashigaru fighting with leased guns.

Due to the changes in fighting styles in these wars, the tachi and naginata became obsolete among samurai, and the katana, which was easy to carry, became the mainstream.

[16] In later Japanese feudal history, during the Sengoku and Edo periods, certain high-ranking warriors of the ruling class wore their sword tachi-style (edge-downward), rather than with the scabbard thrust through the belt with the edge upward.

On the tachi he forged, midare-utsuri (a pattern of hazy white shadows between hamon and shinogi), characteristic of the Bizen school in the Kamakura period.

In general, the signature should be carved into the side of the tang facing outward as the sword is worn on the wielder's left waist.

[38] Unlike the traditional manner of wearing the katana, the tachi was worn hung from the belt with the cutting edge down,[39] and was most effective used by cavalry.

In the late 1500s and early 1600s, many tachi blades were modified into katana, their cut tangs (o-suriage) removing the smiths' signatures from the swords.