An ōdachi (大太刀), meaning "great/thick sword", was a type of Japanese greatsword.

The term nodachi, or "battle sword", which refers to a different type of sword, is often incorrectly used instead of Ōdachi. It is historically known as ōtachi.

The character for ō (大) means "big" or "huge". The character for da (太) and chi (刀) are the same as tachi (太刀), the old style of swords predating the katana.

chi is also the same character as in katana(刀) and the tō for nihontō (日本刀 "Japanese sword"), originally coming from the Chinese character for knife, dāo.

To be considered an ōdachi, the sword in question must have a blade length of 3 chaku (90.9 cm) or more. However, as with many terms in the Japanese sword art, there is no exact definition of the size of an ōdachi.

Ōdachi's Purpose

The purpose of the ōdachi can be categorized as follows:

• As an offering to a temple or to the gods. Some ōdachi were dedicated with prayers to win a war, others were placed in temples as legendary swords from mythology.
• As a weapon. In the explanations of old texts, such as the Heike-momogatari, Taihei-ki tells us that ōdachi were used by soldiers during battles.
• As a symbol for an army. Some ōdachi were too long for practical use. They could not be used in a battle but it is said that they could have been used as a symbol by an army, like a flag.
• As a trend during a certain period. Some swords were also used for ceremonies.
• To show the skill of the forger.
• Used in the front line of battle to attack enemy cavalry, sometimes between two people. It was also used by horsemen to attack infantry.

Most ōdachi were used for the first two reasons.

 

Ōdachi's Production

The ōdachi are difficult to produce because their length makes heat treatment more complicated in the traditional way: the longer a blade is, the more difficult (or expensive) it is to heat the whole blade to a homogeneous temperature, both for annealing and to reach hardening temperature.

Then the tempering process needs a higher tempering medium as uneven tempering can lead to twisting of the blade.

The polishing method 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 that are moved on polishing stones.

Acquiring an ōdachi would be very difficult as they would most certainly have to be custom made. Thus, the local government or a religious organization would have to sponsor production, as there is no reason for their creation otherwise.

Ōdachi's Form of use

The ōdachi that were used as weapons were too long for samurai to carry on their waists like normal swords. There were two methods of carrying them:

• One method was to wear it on the back. However, this was impractical since it was impossible to unsheathe quickly.
• The other method was to simply carry it in the hand. The trend during the Muromachi era was for samurai carrying an ōdachi to have a follower to help them draw.

Swordsmanship styles with ōdachi focused on downward slashes and stances different from those of normal swords.

The importance of the ōdachi faded after the Osaka siege of 1615 (the battle between Tokugawa Ieyasu and Toyotomi Hideyori). From then on it was used more as a ceremonial piece.

  

Ōdachi's Reasons for the loss of popularity

• There were no open field battles after 1615.
• The bakufu government imposed a law prohibiting the possession of swords over a certain length (in 3 of the Genna era (1617), 3 of Kan'ei (1626) and 2 of Shōhō (1645). Once the law was implemented, the ōdachi were trimmed to the smaller legal size. This is one of the reasons why ōdachi are so rare.
• The ōdachi no longer had any practical use, but offerings were still made with them at shinto temples. This became their primary purpose. Because of the amount of skill required to make one, their impressive appearance was considered to make them suitable for praying to the gods.