It is not possible to talk about Samurai without talking about their swords. During the ages they radically changed in form and, at the end, even in meaning. This article will not discuss ideals related to the sword, rather focusing on the change of the shape during the centuries, changes mostly due to practical reasons.
Japan entered the iron age quiet late in front of other civilizations and the China's influence,directly or thru the Korean peninsula, greatly influenced the first swords produced in Japan. Already in the VI cenruty b.C. we have the insurgence of a Japanese taste in fittings but the design of the blades was still strongly influenced by the continent, being the fighting tactics equally imported from China and mainly based on masses of footsoldiers with spears and shields. When these tactics changed due to the fight against Emishi for domain of the Kanto plain, the advantages of a curved sword for horseback fighting begun evident to the japaneses, that already showed their tendence to practicality and skillfullness in adopting foreign items adapting and upgrading them to their needs. Is generally agreed that the fully developed Japanese sword appearance was around the 940 a.C., period in which we find the most ancient extant swords with all the characteristics needed in the "ideal" japanese sword : single edge differentially hardened with strong curvature. Some of these ancient blades already shows another peculiarity of the japanese sword : a softer steel inner core wrapped by one made of harder steel. It's debated if all such ancient blades are made the same way, but at least some shows that this technology was already used in this period.
The japanese swords are divided in periods as follows :
Jokoto 上古刀 pre-938, early Koto 初古刀 938 ~ 1319, middle Koto 中古刀 1319 ~ 1460, late Koto 末古刀 1460 ~ 1596, Keigen-Shinto 慶元新刀 1596 ~ 1624, Kanei-Shinto 寛永新刀 1624 ~ 1658, Kambun-Shinto 寛文新刀 1658 ~ 1684, Genroku-Shinto 元禄新刀 1684 ~ 1764, early Shinshinto 初新々刀 1764 ~ 1818, middle Shinshinto 中新々刀 1818 ~ 1854, late Shinshinto 末新々刀 1854 ~ 1868, Gendaito 現代刀 1868 ~ today.
There is another term we usually find when talking about japanese swords : Shinsakutô 新作刀. This means "recently made swords" and is referred to swords made by a living smith and after 1952 when the 1945 ban of forging swords ended. It's basically a sub-group of Gendaitô, because if the smith pass away, for the NBTHK (Nippon Bijutsu Token Hozon Kyokai) Shinsa (judgement) the sword begins Gendaito.
Another important type of catalogation of japanese swords is according to schools (GoKaDen) and roads that hold the schools.
The first important change occurred after the attempts of mongol invasion in the XIII century. The blades becomes more sturdy on the upper part, the "Ikubi-Kissaki", a sort of very small point, was replaced with other, larger types that left more rooms to repairs after damages.
A second one, only temporary, was the one made in Nanbokucho era, when every type of blade begun greater, longer and heavier, sometimes even too much.
The most important, anyway, occurred in Momoyama era, when the Katana finally replaced the Tachi as main sword of the Samurai. The difference between a tachi and a Katana is, to make a incredible diffucult thing the easiest possible, the position of the Mei (signature). The signature must be on the part of the blade that faces outside. So as tachi is worn edge-down and Katana is worn edge-up, the signatures are placed in opposite positions. Obviously this change was more a low evolution rather then a suddenly revolution. So we have a lot of blades that are "in between" and that is, for now betterr to skip about.
The following layout shows the main (NOT all) "Sugata" (shape) changes of the japanese sword with period and lenght (in shaku, 1 shaku = 30.3022 cm or 11.93 inches) from right to left, first line first. Obviously an infinite number of possible mix are found, but these are the most common ones. When you're reading about a Samurai fighting in the late Kamakura, most likely his sword had the shape you'll find hereunder. Thanks to Valdek Laur for it.
