The emergence of the nihontō, or “Japanese sword” is a particularly noteworthy event in the history of Japanese martial arts. In the Japanese language, the word “tō-ken” (刀剣) is generally used collectively in reference to swords, but “tō” (刀) and “ken” (剣) are in fact different weapons with distinctive morphologies. Tō is a single-edged sword, whereas ken designates a double-edged sword. Thus, tōken is a general appellation referring to different kinds of swords.
The swords used in Japan have their roots in China. During the Spring and Autumn Period of ancient China, in the regions of Wu and Yue, the first swords used as weapons were double-edged. Later, single-edged swords came to replace the double-edged ones on the battlefield, which in turn became implements primarily used in rituals. With this, a differentiation between tō as a weapon and ken as a ritual object was made.
It is said that metallurgical knowledge was brought to the Japanese archipelago at the end of the 3rd century BC, which corresponds with the beginning of the Yayoi period. At that time, as was the case in China, ken were used as ritual objects, whereas the single-edged tō were carried as weapons. The tō inherited from China had straight blades, but the Japanese started to change the form of these weapons and devised their own distinctive style of sword.
The important points that shaped the history of Japanese weapons as follows: First, the Japanese sword was forged with a “sori” or “curvature”. What was the reason for developing a curved blade? When cutting, a straight blade absorbs the impact directly and is easily broken. A curved blade serves to dissipate the shock so is not so susceptible to fracturing, and thus is able to cut better. It is said that this change was influenced by types of sword known as kenukigata–tachi and warabite–tō which were used by the Emishi peoples (now known as the Ainu). They were indigenous people predominantly lived in the north of Japan.
Second, prominent ridges known as shinogi were made on each side of the blade. The simplest blade features a triangle cross-section and is called hira–zukuri; this type of sword is, however, quite weak against lateral shocks, and the probability of fracturing in battle was high, and was inadequate as weapon. In order to improve its functionality, the sides of the blade were fashioned to be parallel with the cutting edge, the cross-section being in the shape of a stretched pentagon: this style is called kiriha–zukuri. It is resistant to lateral shocks and not easily breakable. However, this sword is heavy to wield, and is thus not very suitable as a weapon.
The last improvement was to make the edge more acute than in the kiriha–zukuri, the back of the sword (mine) narrower, and define the shinogi along the sides of the blade. This lighter type of sword is called shinogi–zukuri. The cross-section is diamond shaped, is resistant to lateral shocks, light, and easy to wield.
The evolution from hira–zukuri to kiriha–zukuri happened in China, but the shinogi–zukuri innovation is peculiar to Japan. The characteristics of the swords made in Japan, especially with the sori and shinogi, made them different from the straight blades inherited from China, and this is why they are called nihontō, or “Japanese Swords”. The completion of the Japanese sword happened around the middle of the Heian period in the tenth century. Until then, the way of handling the straight sword presumably focused more on thrusting or pounding the opponent rather than cutting. It is possible that proper cutting techniques were developed after the sori and the shinogi were devised. Nevertheless, it was too early for the nihontō to play a primary role in battle. During these formative years the sword was very much a secondary weapon, as has been pointed out in recent studies on the history of weapons and warfare.