History of the katana through time
Here is an article on the evolution of the katana over time. Obviously it is not an exhaustive study, but is intended to be a simple summary and nevertheless as complete as possible. If you are reading this article, then you are probably an amateur and have heard the term Nihontō more than once. Nihontō does mean Japanese saber, whether old or recent. We tend to call Nihontō the old sabers by "abuse of language" in a way, while Nihontō, should come back more correctly to designate a saber forged in Japan. Indeed, in the Nihontō there are the Jōkotō, the Kotō, the Shintō, the Shinshintō, the gendaitō, showatō, and finally the shinsakutō or shinken, all from a different period, starting with the oldest, until today. You will see that the katana has changed its appearance tremendously over time, sometimes because of wars, but also often in a very interesting and more subtle way, depending on the power that ruled the country.
The Jōkotō
The first weapons appear in Japan as early as the Yayoi period, from 300 BC. The search for efficiency quickly takes place, the blades must be always sharper, and nevertheless solid : do not break easily, and withstand shocks without flinching. The Nihontō, or Japanese saber, which seems to be a highly artistic object, is therefore really issued from a real desire for purely technical improvement. But it turns out that sabers were indeed considered as respected pieces of art very early in the history of Japan, as evidenced by the ancient texts Kojiki and Nihon shoki (8th century). And it does not stop there, since in Japan, the first blades (Jōkotō) were also objects of worship. Thus one of the three sacred imperial equipments is the sword, which is said to have been given by the gods to the imperial lineage to prove its legitimacy. The sacred connotation was further confirmed when weapons were discovered in many very ancient burials, altars, and temples across Japan.

From sword to saber there is a big difference (the sword is two-edged while the saber is single-edged) and the ancestor of our contemporary Japanese saber, or katana, appeared in the middle of the Heian period (794-1099) with the first sabers called Jōkotō (be careful, however, all swords dating from the Yayoi period to the Heian period were also called Jōkotō, but it was during the Heian period that the sabers will later become the katanas that we know). Their shape is indeed very close to the katanas that we still make today. The first curves appearing however towards the middle of the Asuka era (645) with in particular the blacksmith Amakuni from 700 (One of the main figures of the Japanese forge, creator of the tachi). There are then many types of Jōkotō: tsurugi (jian), warabite no tashi, tosu and especially tachis, direct relatives of the katana. The first tachi of the Jōkotō period are not curved, but compared to a tsurugi for example, we can see the connection with the katana, and rather, have a look :

Tsurugi from 5th century, then Jōkotō closed to the katana.
The Nara period (710-794) saw the appearance of many Jōkotō in kissaki moroha-zukuri. Note the double-edged kissaki, inspired by Chinese Jians. The first tachi, attributed to Amakuni Yasutsuna had this form.
The others Jōkotō were generally like those in the previous image, that is to say, with a kissaki (tip) more classical in Kiriha zukuri.

The forge has very much grown during this period, and actually the powers of the government diminished in favor of the clans that divided Japan. Many wars appeared and therefore this explains these technical innovations. Although we then find many Jōkotō of bad qualities (perhaps heating techniques poorly carried out, and poor quality of metals) there are some exceptional sabers which have nothing to envy to modern katanas. Although they seem to be forged from a single piece of metal (and not the fusion of several as we will see later) the grain is excellent, testifying to good quality materials, but also perfectly mastered forging techniques. (The grain, it is motifs visible on the surface of the steel for the eye of an experienced specialist. Not to be confused with damask steel, the motifs of which are due to the different shades of steel). Nevertheless these Jōkotō remain rare.

Here is for example the image of Heishi shorin ken, Jōkotō of excellent quality (now a national treasure or "Kokuho"):
or the Kogarasu-maru, which testifies to the transition between the Jōkotō, straight, and the first Kotō which will become curved :
One among these famous kissaki-moroha zukuri, or kogarasu-maru-zukuri, this last name (so used for the double-edge tip) issued from this saber, is a national treasure forged by Amakuni for the clan Taira.
The Kotō
So it's the turn of these famous Kotō which appear in the second half of the Heian era, up to the Muromashi era. They are again tachis, but this time they become more and more curved : we discover that the result is a better cutting and a better resistance to impacts. Naturally, the blades bended when forging : indeed it is a physical consequence when the edge is close to the back of the blade. Actually, as the steeper and closer is the edge to the back of the blade, so the sharper is the weapon. The blacksmiths had decided no longer to avoid the bends, but on the contrary to work them. The Kiriha-zukuri then give way to the Shinogi-zukuri, an evolution of the point making it stiffer (or thiner) at the level of the cutting edge and the area just after it. The blade is then much sharper (it is the structure still usually used).
Kiriha-zukuri then Shinogi-zukuri
But pay attention : sharper means weaker face to armors when this sharp shape comes from an edge much sharpened (angle more acute), and the risks for breaking are more important. Here is the difficulty of forging Japanese sabers that already appeared at that time: obtaining the right balance, with an ever sharper weapon without compromising its solidity. Playing with angles then becomes essential (later other techniques will make the katanas more flexible and therefore less brittle against shocks, with the coming of composites for example). It needed only a few “failed” Jōkotō with unwanted and poorly corrected curves, as well as good observations, to realize that these curves lead the weapons to be much more incisive. The Kotō was born.

These sabers are not yet called katana, but tachi. There are many other improvements with the Kotō, the quenching lines (hamon) are considerably better done, the nakago (blade tang) are also better made, and excellent sabers are becoming less rare: forging techniques are much better controlled. The first traditions appear with the Yamato Den tradition and its first school, the Senjuin school around 1184, inspired by the blacksmith Amakuni. Then around 1187 the Awataguchi school, taking up the techniques of Sanjo Munechika, allows the Yamashiro tradition to be born, and finally the Bizen tradition to emerge with the Ichimonji school. It is the Bizen tradition that were quickly the most successful, followed closely by the Yamashiro tradition.

Some tachi will subsequently be used mainly for cavalry, with a metal scabbard and a larger sageo (braiding on the scabbard) to avoid friction on the saddle. The blade of the tachi is also much more curved. The optimum impact zone (called "mono- uchi") is therefore located in the first third of the blade, while the middle of the katana blade is the most effective zone. It has an average length of 27,56" and at least more than 23,62", its size is therefore also close to the size of a katana.

Reproduction of tachi from the late Heian era, 11th century:
Example of difference in curve between tachi (left) and katana (right):
Following the Heian era, comes the Kamakura period (1185-1333) when is established a new Shōgun (military government), the first Bakufu ("government under the tent") is set up. This is the period when many blacksmiths are appearing across Japan, developing original and increasingly personal techniques. Schools develop (Bizen, Yamashiro, Yamato, Sōshu) and sabers generally become less sophisticated. They are indeed longer, wider at the level of the tang, and especially more incisive because the kissaki (point of the katana) is lengthened. The tempers are better, the steels more resistant, and the Soshu tradition which is largely at the base of these modifications meets a great success (tradition born of the blacksmith Yukimitsu member at the beginning of the Yamashiro tradition). This can be explained by the fact that the military power is more severe and aggressive than before, and that the country is going to war with the Mongols. In fact two Mongol invasions have failed, but a third seems to be looming. It will ultimately never take place, but the announcement of this possible third attack, and the feeling of a state of siege provoked by the two preceding ones, considerably reached the world of the forge. The creation of new hamon (quenching lines) for example are appearing, simpler and in a straighter line. A priori they would have been more solid, but many people claim that these hamon are not actually more resistant. Only a few blacksmiths, referred to as "the steadfast blacksmiths" continued producing sabers in accordance with the shape of the late Heian era, and the early Kamakura era. They even made some technical and styling changes.
japan katana saber
Schematics of the book "The connoisseur's book of japanese swords"
The Nanboku-chō era (1333-1392) at last shows different long blades, which were up to that time only tachis. The nodachi are appearing, but it is above all a remarkable period because the beautiful and already imposing forms of the Kamakura period are taken again to be even more enlarged. The sabers then become very imposing, and their excellent technical qualities are not at all jeopardised. This is also when the last of the 5 traditions will appear, or Gokaden, with the Mino tradition, coming from the Tegai school of the Yamato tradition with Kaneuji. Kaneshige (or Kinju) from Echizen province will also be a great figure in the early days of the Mino tradition. The early Mino were considered to be a mix between the Sōshu and Yamato traditions. The success of the Sōshu tradition is confirmed, and this tradition will attract very great blacksmiths such as Masamune or Sadamune. The Yamato and Yamashiro traditions are in decline, and eventually merge. The Muromachi era (1392 1573) finally saw the disappearance of the Kotō-type tachi which were then largely replaced by the katanas. Indeed the fights change, and it becomes important to be able to draw and strike at the same time (Nuki Uchi). The tachi, which is too curved and mainly too long, does not allow this maneuver and therefore it is carried sharp downwards, while the katana is carried sharp upwards. The blades lose the aggressiveness they had acquired during the Nanboku-chō period to get closer to what was made in the Kamakura era, with blades of 27,56"- 28,74"(standard size called Josun), even seen 23,62" during the Sengoku period. For sabers, this period is divided between before the Ōnin war (civil war that will last 10 years), and after the war. During the first part we have sabers which are confusing a little bit : tachi and katana are very close to each other, and sometimes it is even only the signature that differentiates them upon request of the guy who ordered the blade, tachi or katana being noted on the tang of the blade (katana-mei and tachi-mei). The Uchigatana appears in 1429, it measures only 23,62" on average and no more than 27,56", and can therefore easily be used with one hand. The wakizashi Shintō which will come later will nevertheless stand out from the Uchigatana, which will disappear rather quickly, just like finally the tachi, to give way to the terms katanas and wakizashis. The term katana then makes it possible by extension to designate the Uchigatana and the Tachi. Even today, however, we can distinguish certain tachi in the katana category, when they are truly intended for cavalry and their appearance makes it largely notable.

Often of rather poor quality, generally intended for low-ranking warriors, we begin to distinguish the sizes and to give the names katanas and wakizashis in common use. The term Uchigatana will disappear over time to make room for these new terms. Poor quality indeed, because the civil war of Ōnin generated a mass production, and the famous blacksmiths are less and less numerous. Katanas are produced in the simplified Bizen and Mino tradition, and are called kazu-uchi mono or tabagatana (mass-produced sabers), of lower quality, differentiated from chumon-uchi (high-quality, custom-made sabers, generally made for the Lords).

Composite forging continues to develop with the appearance of kobuse and makuri applied to katana, blades made of different grades of steel, to have a hard surface and a softer and flexible blade interior for better shock absorption.
katana steel japan
It is also during this period that the musket arrives in Japan, brought back by the Portuguese in 1543. A large production is then ordered, and more than 3000 muskets will be made for 10 years.
The Shintō
Then is the Azuchi Momoyama era, with the arrival of the unifying leaders to power (Nobunaga Oda, Toyotomi Hideyoshi, and Tokugawa Ieyasu). Many modifications will allow the craft in general to develop enormously. Unification eliminates the blacksmith schools (the five great traditions), but great masters are appearing all over Japan. In addition, kenjutsu and the wearing of daishō are developing very much. We speak about Shintō ("new sabers") which are very different from what was done before. The term is first used in "Shintō Bengi", a book published during the An' era (1772 1781) by Kamata Natae. Nevertheless, the quality of the grain of these Shintō is generally less good than that of the Kotō. This is due to the massive importation of lower quality steel from Portugal and Holland (nanbantetsu or even hyotantetsu or konohatetsu as opposed to watetsu , Japanese steel), and the use of poor quality Japanese steel from the west (steel containing too much phosphorus, which increases the risk of blade breakage). But the after period of mass production during the Ōnin war shows clearly that the term Shintō is justified : these new sabers are of an undoubtedly superior quality concerning the forging techniques which have been developed enormously since the end of the Muromachi era, in parallel with this rapid and massive production. The old techniques have disappeared, but are rediscovered step by step, and new techniques are added to create a forging art of excellent quality. Art mainly created by Umetada Myoju, who is considered to be one of the founders of the Shintō, without being sure of his involvement since he seemed above all to be a great blacksmith of tsuba, menuki, fuchi, tsuka-gashira, kogai and kogatana). Valuations (Kantei) are becoming more common, and so the Hon'ami family has created a first classification of blades. Certificates were awarded, and the katanas of the Sōshu tradition had a prominent place in this classification.

The Edo era, also sometimes called the Tokugawa period, shows the reappearance of a desire to make aesthetic and refined katanas as the Kotōs were. We still talk about Shintō, although there is therefore a big difference between the first Shintō, and the latter, which is in addition more aesthetic are also often of a better quality (forging techniques having continued to develop). The curves are revised to imitate not so much those of the tachi, and magnificent hamons (quenching lines) are elaborated. These tempers are known as Shinto Tokuden, it is a new art. A difference between east and west appears, we speak of Osaka Shintō and Edo Shintō. Osaka being a cultural city, we have more sophisticated katanas, while in Edo, a new city where the Bushidō code is extremely strict, we often have more imposing katanas, the technical qualities of which takes precedence over aesthetics. We then speak of wazamono to designate the katanas that cut very well, since it is a period during which the cutting test (tameshigiri) is current, and some people are officially employed by the government to do these tests. Then, they are entitled to try it on criminals and prisoners, and we can see engraved on the silk (tang) of the blade of the katanas what we will call tameshi-mei such as "Futatsu do otosu" (has cut two trunks ). These tests can significantly increase the price of a blade, note that katanas during this period are more and more expensive : in the end blacksmiths obtain an ever greater social status. Decrees will appear during this period, the first in 1638 to impose a maximum size of 33.92" for a katana, and 20.56" for a wakizashi (size of the blade starting from the habaki). The large katanas are then really put aside. In 1712 these sizes are reviewed with a new decree: 35.04" for a katana, 21.80" for a wakizashi.
The Shinshintō
The end of the Edo era and the beginning of the Meiji era (1781-1876) saw the birth of the Shinshintō, within a period as the powers were returned to the emperor by the Shōgun (because his power was decreasing in favor of the merchants and other members of the people) and as peace took place, the katana seemed to be forgotten. Fortunately, Suishinshi Masahide, a great actor from the Shinshintō period, will bring the katana back to life. Coming from a family of samurai, he will train a hundred students in Edo. Thanks to him the forge is alive again, the tachi reappears, as well as the five Gokaden (blacksmith schools). The Shinto Tokuden is still practiced. It is a period during which katanas very different each other from one another are forged, with the idea to ​​rediscover the different traditions, Kotō included. The Tanto reappear, whereas they seemed to have disappeared during the Shinto period. Over time, a large part of the blacksmiths will try to find the styles of the Kamakura and Nanboku-chō eras, when magnificent blades had been forged. So we cannot really speak of a very specific style for this period, blacksmiths are also beginning to mix styles, forging katanas from one school and then another. The blacksmiths were not specialized and so it was not always a success. With the Meiji era comes the disappearance of the samurais (Haitorei decree prohibiting the carrying of weapons in 1868). The katana then becomes truly an object of art, and as soon as 897, some of them are declared national treasures.
The showato
The showatō will appear during the Showa period during the Second World War: they are the last katanas for realy military use. Many do not respect traditions, and are of poor quality. Nevertheless a few sabers stand out such as those of the Yasukuni forge center, the katanas designated by Rikugun Jumei Tosho, and those of Gassan Sadakatsu, or Nobufusa. The katanas will also aim to give courage to men during modern wars. The goal was to produce more than 2,150,000 sabers to equip the men of the Japanese army each of them ought to have one. They are called "guntō", Japanese sabers produced in large quantities for military use. At last, it is estimated that 2,000,000 sabers were created during the Kotō, Shintō, and Shinshintō periods, so the challenge was impossible since even 900 years were not enough to produce so many sabers. But in order to get as close as possible to this goal, the traditional steel (Tamahagane) was abandoned to produce all these katanas for a much cheaper industrial steel, and simpler techniques of mass production. Designed for modern warfare, they were largely metallic (Saya or iron scabbard, Tsuka or copper handle...). We will talk about Shin-gunto for the army, Kai-gunto for the navy, as well as Kyu-gunto for the closed grips as the European way. When the Japanese surrendered, the United States ordered the destruction of these sabers to humiliate the Japanese people. Fortunately, many of these historic katanas were saved. Finally, showatō and guntō refer to katanas made at least not traditionally, and often by a production line. Good quality katanas from this period are called gendaitō.
Kyu-gunto (European grip)
katana saber
The gendaitō and shinsakutō or shinken
The gendaitō or modern saber, is the name given to katanas forged after the last modification of the Haitorei edict in 1876 (ban on the wearing of the katana), until 1945. Unlike showatō and guntō produced during the same years, gendaitō are considered to be of good quality because they are forged according to tradition. The wearing of the katana being prohibited, the production has decreased dramatically. The demand having almost disappeared, the blacksmiths have move to another job. So much so, that the emperor, keen for katanas (but having taken these measures for political reasons) has appointed GIGEI-IN, or craftsmen, that he asked to return to their original activity and they were paid accordinglyl. This is how Gassan Sadakazu and Myamoto Kanenori were named GISEI-IN in 1906 by the emperor. Today, katanas forged by accredited Japanese masters are called shinsakutō or shinken. The katanas are objects of art, and in addition they really keep their cutting abilities as those made in the past.

Currently the katana has a bright future ahead, among other reasons, thanks to the NBTHK (NIHON BIJUTSU TOKEN HOZON KYOKAI), an organization for the protection of traditional katanas, created by specialists, collectors and blacksmiths in 1948, which has rehabilitated the large Tatara furnace in Shimane, Japan. Today it can produce about two katanas per month, using the most traditional Tamahagane steel. They are obviously forged by great masters. These are usually soshu den and bizen den. Descendants of great blacksmiths are still active and they pass on their art as well as their predecessors. We have recent katanas of which qualities are as excellent as for many Shintō or Shinshintō. More, we think that sometime we will be able to reach the Kotō quality of Masamune from Soshu, or of Ichimonji from Bizen. The Japanese saber is therefore not at all going to be lost, despite the mass production (which already existed several hundred years ago). The artistic katana, the best technically, is still relevant today, and the techniques are constantly being transmitted and improved.
Shinsakutō (recent and functional katana) by Fukuda Yoshimitsu