Japanese Sword expert Paul Martin's simplied description of swords through the ages…
Jõkotõ are straight blades with no curvature. They are usually constructed in the hira-zukuri and kiriha-zukuri styles. It is thought that the shift from straight blades to Japanese swords with curvature happened around the mid to late Heian period (794-1184). This was during the mid 10th century: about the time Taira Masakado and Fujiwara Sumitomo rebelled against the government in the Johei (931-938) and Tengyo (938-947) eras. Blades before these are continental style blades called jõkutõ, and were brought to Japan from the Asian continent. Examples of these blades have been excavated from Kofun period tombs, and some still reside in the Shosoin Imperial Repository, Nara.
1. Late Heian to Early Kamakura
From the late Heian period and the early Kamakura period (1185-1333) we can see the Japanese sword as we [now] know it: shinogi-zukuri (ridgeline) construction, with a wide base, narrowing acutely towards the small point section (ko-kissaki). They are quite slender blades with the curvature concentrated between the handle and base. This shape is called koshi-zori. From midway towards the point there is very little curvature. These blades are usually around 2.5-6 shaku in length (75.8 cm-78.8 cm)
At the zenith of the warrior class’s power during the Kamakura period, the blade’s kasane becomes thick, the mihaba becomes wide and they take on magnificent tachi shape. There is not much difference between the size of the moto-haba and the saki-haba. The blade still has koshi-zori, but the center of the curvature has moved further along the blade. The kissaki has become a compact chu-kissaki (ikubi). The hamon has developed into a flowing gorgeous choji-midare. Also around this time, tanto production appears.
3. Late Kamakura
Tachi at the end of the Kamakura period have developed into magnificent blades. There are two types: one is wide throughout its length and the point section is the same as mid- Kamakura period kissaki, but slightly extended. The other is quite slender and similar in appearance to the late Heian, early Kamakura shapes. However, when you look further along the blade the shape has changed; the curvature has moved further along the blade. During this period notare-gunome hamon appeared. It is said that in Sagami province Goro Nyudo Masamune perfected the production of nie-deki blades.
During the Nanbokucho period many long blades of three shaku (90.9cm) and other long tachi were made. Tanto of large proportions were also produced. Tachi were majestic, wide, and proportionally long. Among these were some over 90 cm in length and worn over the back. These types of blade are called no-dachi and o-dachi. They were rather thin in construction to decrease the weight. Additionally, many have a bo-hi (groove) cut into the shinogi-ji area in order to lighten the blade. Many tachi from this period are o-suriage (shortened in later periods as they were difficult to wield). Consequently, many extant blades from the Nanbokucho period are unsigned.
5. Early Muromachi
Blades of the early part of the Muromachi period are reminiscent in construction to the blades of the early Kamakura period. When compared to the shape of the Nanbokucho period, the shape has completely changed and no longer includes o-kissaki. At around 2.4-5 shaku (72.7 cm-75.7 cm) in length, they are quite narrow and deeply curved with a medium-sized point section. At first glance they may appear somewhat similar to Kamakura period blades, but on closer inspection they are saki-zori character.
6. Late Muromachi
By the late Muromachi period, samurai fighting methods had changed from cavalry to mass infantry style warfare. Uchigatana, worn with the cutting edge uppermost thrust through the sash, had become popular. After the Onin war, conflicts broke out in many places and kazu-uchi mono began to appear (mass-produced blades inferior in quality to regular Japanese blades). However, specially ordered blades of excellent quality (chumon-uchi) were also produced at this time. Bizen (Okayama prefecture) and Mino (Gifu prefecture) were the major places of production. Many blades produced in this period are around 2.1 shaku (63.6 cm) in length. They are slightly wider than the standard width, with either a chu-kissaki or an extended chu-kissaki and strong saki-zori. The nakago are short, intended for one-handed use.
Swords produced up to the Keicho era (1596-1614) are classified as kotõ (old blades); blades made during and after this era are classified as shintõ (new-swords). When Japan entered the Azuchi-Momoyama period, many smiths moved to Edo or Kyoto, or gathered in castle towns of various influential daimyo. Additionally, developments in transportation brought about experimentation with materials, and foreign-made steel (known as nanbantetsu) was utilized. The blade’s shape from around this period mirrors that of shortened Nanbokucho blades. They are generally wide with little or no difference between the moto and saki-haba. Many have an extended chu-kissaki, whilst some have o-kissaki, with a thick kasane and are usually around 2.4-5 shaku (72.7 cm-75.8 cm) in length.
Swords of the mid-Edo period are of standard width. The saki-haba is relatively narrow when compared to the moto-haba. The curvature is noticeably shallow with a small to medium-sized point section. They are usually around 2.3 shaku (69.7 cm) in length. This particular type of construction was generally produced around the middle of the Kanbun (1661-1673) and Enpo (1673-1681) eras, and is usually referred to as Kanbun shintõ.
9. Edo Period Genroku era
The change in shape of Japanese swords between the Jokyo (1684-1688) and Genroku (1688-1704) eras reflects the transition of shape from Kanbun-shintõ blades to the beginning of the shin-shintõ period of sword manufacture. As it was a very peaceful period in Japanese history, rather flamboyant hamon appear, and as opposed to that of Kanbun-shintõ blades, the curvature is quite deep.
10. Edo period Bakamatsu era
Blades made after the Bunka (1804-1818) and Bunsei (1818-1830) eras are referred to as fukko-shintõ (revival swords). Pioneers of the revival movement include Suishinshi Masahide and Nankai Taro Tomotaka. Taikei Naotane was among Masahide’s students. Minamoto Kiyomaro led a revival aimed at soshu-den and Mino-Shizu workmanship. Bakumatsu blades are shallow in curvature, have a wide haba with not much difference in width between the saki and moto-haba, and are around 2.5-6 shaku (75.7cm-78.7 cm) in length, with an o-kissaki and thick kasane.
11. Meiji Onwards (no image)
Blades made from the 9th year of Meiji until present day are referred to as gendaitõ (modern swords). As of the Hatorei decree in 1876 (banning civilians from wearing swords), the need for swords declined. However, in Meiji 39 (1906) the craft gained imperial patronage. The swordsmiths Gassan Sadakazu and Miyamoto Kanenori were appointed Tei Shitsu Gi Gei In (craftsmen by imperial appointment—equivalent to National Living Treasure). Since then, the swordsmith’s craft has continued through the Meiji (1868-1912), Taisho (1912-1926), Showa (1926-1989) and Heisei (1989-) eras until today. Today’s swordsmiths try to recreate the workmanship of eminent smiths of every period, regardless of whether they are kotõ, shintõ or shin-shintõ. In particular, recreations of tachi of the Kamakura period are a popular aim for many modern swordsmiths.
For more information about Japanese swords, see Paul Martin's website: http://www.thejapanesesword.com/