An ?dachi () (large/great sword) or nodachi (, field sword) was a type of traditionally made Japanese sword (, nihont?) used by the samurai class of feudal Japan. The Chinese equivalent and 'cousin' for this type of sword in terms of weight and length is the miao dao, and the Western battlefield equivalent (though less similar) is the longsword or claymore.
The character for ? (?) means "big" or "great". The dachi here () is the same as tachi (, lit. "great sword"), the older style of sword/mounts that predate the katana. The chi is also the same character as katana (?) and the t? in nihont? ( "Japanese sword"), originally from the Chinese character for a blade, d?o.
To qualify as an ?dachi, the sword in question would have a blade length of around 3 shaku (90.9 centimetres (35.8 in)); however, as with most terms in Japanese sword arts, there is no exact definition of the size of an ?dachi.
Practically speaking, the function/use of most ?dachi fall into the first two categories--as ceremonial objects and infantry swords. The possible functions of the ?dachi can be categorized as follows:
?dachi are difficult to produce because their length makes traditional heat treatment more complicated: The longer a blade is, the more difficult (and expensive) it is to heat the whole blade to a homogeneous temperature, both for annealing and to reach the hardening temperature. The quenching process then needs a bigger quenching medium because uneven quenching might lead to warping the blade.
The method of polishing is also different. Because of their size, ?dachi are usually hung from the ceiling or placed in a stationary position to be polished, unlike normal swords which are moved over polishing stones.
In the past, acquiring a fully sharpened ?dachi was difficult, as they required special custom orders. In modern times though, many forges in Japan and China are accepting such orders due to the renewed popularity and interest in ancient Japanese weaponry in the late 20th and early 21st centuries.
As battlefield weapons, ?dachi were too long for samurai to carry on their waists like normal swords. There were two methods in which they could be carried: One was to carry it on one's back. However, this was seen as impractical as it was impossible for the wielder to draw it quickly. The other method was simply to carry the sheathed ?dachi by hand. The trend during the Muromachi era was for the samurai carrying the ?dachi to have a follower to help draw it.
An exception does exist, though. The K?den Enshin-ry? taught by Fumon Tanaka use a special drawing technique for "short" ?dachi allowing it to be carried on the waist. The technique is to pull out the sheath rather than drawing the blade. While this move is also used in other schools, for example, Yagy? Shinkage-ry?, Shin mus? Hayashizaki-ry? and Iaid?, only Enshin-ry? seems to have used it to improve the drawing speed of an ?dachi, the other schools having used it with classical katana. The Kage-ry? style is also used to draw from the belt, using blades of approximately 2.8 shaku.
?dachi swordplay styles differed from that of other Japanese swords, focusing on downward cuts.
One possible use of ?dachi is as large anti-cavalry weapons, to strike down the horse as it approaches. Alternatively, it could be used as a cavalry-on-cavalry weapon comparable to the Chinese zhanmadao, with the long reach, increased weight and slashing area of the blade offering some advantages over spears, lances and smaller swords.
This loss of popularity is due to the Shogunal government setting a law which prohibited holding swords above a set length (in 1617, 1626, and 1645). After the law was put into practice, ?dachi were cut down to the shorter legal size. This is one of the reasons why ?dachi are so rare. ?dachi were no longer of practical use, but were still made as offerings to Shinto shrines. This became their main purpose. Due to the amount of skill required to make one, it was considered that their exotic appearance was suitable for praying to the gods.