A Japanese Sword, or nihontō, is one of the traditional bladed weapons of Japan. These are categorised in several types according to size and method of manufacture. The most commonly known type is the katana, which, like the similarly formed tachi, is a single-edged and, usually, curved long sword which was traditionally used by samurai from the 1400s onwards; Wakizashi is the shorter sword; Tsurugi are double-edged long swords; Ōtachi or Nodachi are older but longer single-edged versions.

Although they are pole-mounted weapons, the Naginata and Yari are still considered part of the Nihonto family due to the methods by which they are forged.cJapanese swords are still commonly seen; antique and modernly-forged swords can easily be found and purchased. Modern, authentic nihontō are made by a few hundred swordsmiths. Many examples can be seen at contests hosted by the All-Japan Swordsmiths Association.

Before 987, examples of Japanese swords were straight chokutō or jōkotō and others with unusual shapes. In the Heian period (8th to 11th centuries) sword-making developed through techniques brought over from China through trade in the early 10th century during the Tang Dynasty and through Siberia and Hokkaidō, territory of the Ainu people. The Ainu used warabite-tō and these influenced the nihontō, which was held with two hands and designed for cutting, rather than stabbing.

According to legend, the Japanese sword was invented by a smith named Amakuni (ca.700 AD), along with the folded steel process. In reality the folded steel process and single edge swords had been brought over from China through trade in the early 10th century during the Tang Dynasty. Swords forged between 987 and 1597 are called kotō; these are considered the pinnacle of Japanese swordcraft. 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.

Almost all blades are decorated, although not all blades are decorated on the visible part of the blade. Once the blade is cool, and the mud is scraped off, the blade may have designs or grooves (hi or bo-hi) cut into it. One of the most important markings on the sword is performed here: the file markings. These are cut into the nakago or the hilt-section of the blade, where they will be covered by the tsuka later. The nakago is never supposed to be cleaned: doing this can reduce the value of the sword by half or more. The purpose is to show how well the blade steel ages.

Source: Wiki