Ōdachi

Practically speaking, the function/use of most ōdachi fall into the first two categories—as ceremonial objects and infantry 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 used in prayer before a war, while others were displayed (sometimes in temples)—reputedly as legendary swords from mythology.
• 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.

A sheathed ōdachi

Ōdachi 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, 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.

In the past, acquiring a fully sharpened ōdachi 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.

Samurai with an ōdachi on a horse

Edo period wood block print showing an ōdachi being worn on the back of a samurai.

As battlefield weapons, ōdachi were too long for samurai to carry on their waists like normal swords. There were two main 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 ōdachi by hand. The trend during the Muromachi era was for the samurai carrying the ōdachi to have a follower to help draw it.

An exception does exist, though. The Kōden Enshin-ryū taught by Fumon Tanaka use a special drawing technique for "short" ōdachi 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, Yagyū Shinkage-ryū, Shin musō Hayashizaki-ryū and Iaidō, only Enshin-ryū seems to have used it to improve the drawing speed of an ōdachi, the other schools having used it with classical katana. The Kage-ryū style is also used to draw from the belt, using blades of approximately 2.8 shaku.

Ōdachi swordplay styles differed from that of other Japanese swords, focusing on downward cuts.

One possible use of ōdachi 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.

The ōdachi's importance declined 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 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, 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.

• Greatsword

Wikimedia Commons has media related to Odachi.

• Richard Stein's Japanese sword guide
• Ōdachi Tanto Blade Benefits
• Pictures of some blades