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An ōdachi (大太刀) (large/great sword) was a type of long traditionally made Japanese sword (nihonto) used by the samurai class of feudal Japan. The term nodachi (field sword) refers to the same type of sword.
The character for ō (大) means "big" or "great". The characters for da (太) and chi (刀) are the same as tachi (太刀, lit. "great sword"), the older style of sword/mounts that predate the katana. The chi is also the same character as katana (刀) and the tō in nihontō (日本刀 "Japanese sword"), originally from the Chinese character for a knife, dāo.
To qualify as an ōdachi, the sword in question would have a blade length of around 3 shaku (35.79 inches or 90.91 cm) however, as with most terms in Japanese sword arts, there is no exact definition of the size of an ōdachi.
Practically speaking, the function/use of most ōdachi fall into the first two categories—as ceremonial objects and cavalry swords. The possible functions of the ōdachi can be categorized as follows:
- As a votive offering to a shrine (or specifically to its patron gods). Some ōdachi were use in prayer before a war, while others were displayed (sometimes in temples)—reputedly as legendary swords from mythology.
- The average length of an ōdachi is 65–70 inches long (approx 165–178 cm), often with a 4–5 foot blade. This made them unsuitable for close-quarters combat. Instead, they are commonly believed to have been used by fighters on horseback, as the blade length would allow them to take down infantry (without risk of being pulled off their mount).
- Like other trends, ōdachi were often in vogue, most notably during the Edo Period, so it was not uncommon to see the swords used in various ceremonies.
Ōdachi are difficult to produce because their length makes heat treatment in a traditional way more complicated: The longer a blade is, the more difficult (or expensive) it is to heat the whole blade to a homogenous 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, Ōdachi are usually hung from the ceiling or placed in a stationary position to be polished, unlike normal swords which are moved over polishing stones.
Acquiring a fully sharpened Ōdachi would be hard as they would almost certainly have to be custom-made.
They are still made on occasion. One such sword, made in 1971, (now in a private collection in Texas) was made at the request of a dying man. Its purpose was to please the gods and enlist their protection of his family after his impending death.
Method of use 
Ōdachi that were used as weapons were too long for samurai to carry on their waists like normal swords. There were two methods in which they could be carried: One method 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 ōdachi by hand. The trend during the Muromachi era was for the samurai carrying the ōdachi to have a follower to help draw it.
Ōdachi swordplay styles focused on downward cuts and different wields than those of normal swords.
Reasons for loss of popularity 
The ōdachi's importance died off after the Siege of Osaka of 1615 (the final battle between Tokugawa Ieyasu and Toyotomi Hideyori). Since then, it has been used more as a ceremonial piece.
This loss of popularity is due to:
- The Bakufu government set a law which prohibited holding swords above a set length (in Genna 3 (1617), Kan'ei 3 (1626) and Shōhō 2 (1645)).
After the law was put into practice, ōdachi were cut down to the shorter legal size. This is one of the reasons why ōdachi are so rare. Ōdachi 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.
See also 
- The Development of Controversies: From the Early Modern Period to Online Discussion Forums, Volume 91 of Linguistic Insights. Studies in Language and Communication, Author Manouchehr Moshtagh Khorasani, Publisher Peter Lang, 2008, ISBN 3039117114, 9783039117116 P.150
- The Complete Idiot's Guide to World Mythology, Complete Idiot's Guides, Authors Evans Lansing Smith, Nathan Robert Brown, Publisher Penguin, 2008, ISBN 1592577644, 9781592577644 P.144
- Classical weaponry of Japan: special weapons and tactics of the martial arts, Serge Mol, Kodansha International, Jun 17, 2003 P.17
- Samurai fighting arts: the spirit and the practice, Fumon Tanaka, Kodansha International, Jun 16, 2003 p.12
- State of war: the violent order of fourteenth-century Japan, Thomas Conlan, Center for Japanese Studies, University of Michigan, 2003 P.260
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