The oldest Japanese swords were generally straight-bladed. The first curved swards (tachi) appeared during the Heian period. Later katana, the most famous Japanese sword, came in the stead of tachi. Tachi and katana blades are equal in length: about 70-80 cm. Both types of swords had a grip long enough (ca. 20-25 cm) to accommodate one or two hands. The main difference of the two types was the way the swords were worn. The tachi was worn with the cutting edge down and the katana was worn with the cutting edge up. Traditionally the signature should be carved into the side of the blade that would face outward when the sword was worn. That’s why katana and tachi can be easily distinguished from each other by the location of the signature.
However, there were some little structural differences. Tachi blade is believed to be more curved than katana blade, but that’s not always the case.
There existed other types of swords. A smaller wakizashi (with a blade of only 45 cm in length) was often worn with the katana – such pairing is called the daisho. Wakizashi was normally used not in battles but in situations, in which the longer katana was not suitable. Moreover it was wakizashi that was often used for commiting seppuku (a ritual suicide).
Large two-handed sword was called nodachi. Nodachi was about 120 cm in length. Because of such a length, the sword was worn behind the back. In spite of its impact force, much more powerful than that of katana or tachi, nodachi was rarely used. First, such a long blade was much more difficult to forge than katana or tachi, that’s why it was very expensive. Second, the heavy and long nodachi was considerably hard to use: only very experienced samurai could use them during battles.
The construction of samurai sword was a hard and long process, and that made the swords very expensive. The appearance of the numerous ashigaru armies during the Sengoku period called forth the need for the mass production of swords. Surely, such cheaper swords lacked the quality of samurai swords (a situation typical for ashigaru equipment). The Edo era brought the end to wars and, consequently, to the production of cheap swords. However, the history repeated itself: in XX century the sword mass production for the Japanese army began anew – this time factory-made. Of course those swords were not of better quality than their Sengoku era predecessors.
Taken from the original website by ZVEZDA.
http://samurai-battles.com/historical-reference/medieval-japanese-swords/