on horseback; the curve of the blade adds considerably to the downward force of a cutting action. According to historian Karl Friday, before the 13th century, there are no written references or drawings showing swords of any kind were used from horseback. 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. Early models had uneven curves with the deepest part of the curve at the hilt. As eras changed, the center of the curve tended to move up the blade. By the 11th century during the Heian period, tachi were exported to neighboring countries in Asia. For example, in the poem "The Song of Japanese Swords" Ouyang Xiu, a statesman of the Song Dynasty in China, described Japanese swords as "It is a treasured sword with a scabbard made of fragrant wood covered with fish skin, decorated with brass and copper, and capable of exorcising evil spirits. It is imported at a great cost.". From the Heian period (794–1185), ordinary samurai wore swords of the style called kurourusi tachi (kokushitsu no tachi, 黒漆太刀), which meant black lacquer tachi. The hilt of a tachi is wrapped in leather or ray skin, and it is wrapped with black thread or leather cord, and the scabbard is coated with black lacquer. On the other hand, court nobles wore tachi decorated with precisely carved metal and jewels for ceremonial purposes. High-ranking court nobles wore swords of the style called kazari tachi or kaza tachi (飾太刀, 飾剣), which meant decorative tachi, and lower-ranking court nobles wore simplified kazatachi swords of the style called hosodachi (細太刀), which meant thin tachi. 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. Since tachi worn by court nobles were for ceremonial use, they generally had an iron plate instead of a blade. In the Kamakura period (1185–1333), high-ranking samurai wore hyogo gusari tachi (hyogo kusari no tachi, 兵庫鎖太刀), which meant a sword with chains in the arsenal. The scabbard of the tachi was covered with a gilt copper plate and hung by chains at the waist. At the end of the Kamakura period, simplified hyogo gusari tachi came to be made as an offering