Iaido is practiced with the Japanese sword. This weapon has, for hundreds of years, been considered both a weapon and a work of art. The first swords in Japan were straight double- and single- edged weapons called ken, likely created on the Asian mainland. Around 700 A.D. a native form called the tachi was created which was single-edged and curved. A relatively long, light blade useful for fighting on horseback, the tachi was worn on the left side, edge down, suspended from cord hangers. By the mid-1500s a shorter, more robust blade called a katana had became popular. This was worn edge up and with the scabbard through the belt rather than hung on a cord. The katana was mostly used two-handed but was otherwise similar to the tachi.
The Japanese blade was forged in a specialized manner. Pieces of smelted iron from a charcoal furnace were hammered and folded repeatedly using a charcoal fire.This created a mostly homogenous piece of steel of a certain carbon content. Several pieces of steel were then placed together and forge welded so an inner core of low carbon was surrounded by a layer of high carbon steel. The blade was then hammered into shape.
The blade was coated with clay, thicker along the back and thinner at the edge, then heated and cooled in water. The edge cooled quickly and the back more slowly, thus creating a blade with small, hard crystals of metal on the edge and larger crystals on the back. The differing carbon contents and crystal type created a blade with a very hard (brittle) edge that would resist dulling, and a relatively more flexible back to prevent the blade from shattering on impact.
During many years of conflict, the sword was considered a secondary weapon on the battlefield, behind the bow or pole weapons such as the spear. If the primary weapon was lost, it was necessary to get the sword into play quickly and this is often seen as the beginning of Iaido techniques. During the Tokugawa period (1603–1868) the Samurai class ruled the country and one of their symbols of rank was wearing two swords. The sword thus became the primary weapon of budo and hundreds of sword schools were developed, many surviving into the modern era. During this time an appreciation of the sword as an art object began, which continues today with thousands of sword collectors worldwide.