This writeup aims to provide a short summary of the wide and varied styles present in Japanese sword construction, in order to further the recognition and understanding of the history of this ancient art. If indeed you wish to attend a Japanese sword show, you will see each of these styles and probably even stranger ones. Reading through these simple styles will save you gawking like an idiot the first time you see a double-edged katana; or worse, having to ask what kind of sword it is and making an idiot of yourself. This writeup will focus primarily on the katana longsword, as most variety is seen in the construction of this weapon.
As stated in its home node, the katana is a long Japanese sword, the primary armament of the samurai warrior. When asked to describe a katana, most people might mention its single-edged, curved-blade design. Perhaps the technique of differential tempering used to create a Rockwell gradient from front to back, and the resultant hamon temper-line. Those exposed only to Western interpretations of the katana might note the chisel-like point, supposedly for armour-piercing purposes.
Not one of the above characteristics are true for all, or even most, katana. Like any country, Japan's martial history is both long, bloody, and varied. The blade we know today as the katana has gone through numerous changes in it's history, which has in the fullness of time produced a wide variety of schools and styles. Just as European swordsmiths have over time evolved the almost Roman-styled shortsword into hand-and-a-half and two-handed broadswords, so their Japanese counterparts evolved the katana from a simple straight-bladed and rather heavy sword into the speedy, sharpened death banana we all know and love.
The chokuto bladeform is among the earliest examples of Japanese sword forging. It is indeed a straight-bladed variant of the katana, and was used before the development of differential tempering technology. Generally, the chokuto underperforms against the lighter, curved blades of later years, hence the paradigm shift from straight to curved in the samurai culture. Later, very skilled swordsmiths discovered ways to differentially temper a chokuto blade whilst preserving it's straightened form - the clay process is what gives a modern katana it's curve. Although less useful in battle, it was considered a greater test of the swordsmith's art to produce a good chokuto, due to the difficulty in countering the natural curvature. As an aside, the straight-bladed Hollywood 'ninja-to' was probably based on the chokuto, although no historical records actually indicate that a ninja would ever advertise himself by carrying such a distinctive blade. The tip of a chokuto blade is curved, delimited by a ridge perpendicular to the blade, a couple of inches from the point. Chokuto also usually have a ridge running the length of the blade, one-third of a blade depth down from the spine.
This is the most common bladeform seen, both in ancient and modern katana forging. shinogi-zukuri blades are long, curved swords bearing the traditional ridgeline along the length of the blade. A shinogi blade will also have another ridgeline perpendicular to the blade near the tip, defining where the arc of the point begins. Referring back to the comment on triangular or 'chisel-points' made in the opening paragraphs, this is not seen in any variant of Japanese sword. It is entirely the invention of Western movie producers, presumably because it is easier to make a straight point than a curved one. In shinogi blades, the point is merely follows a tighter curve than the rest of the blade, the start of which is defined by the second blade ridge. Being the most common bladeform, there is naturally the greatest degree of sub-varieties in the shinogi category. Most blades have the common boshi cross-section, but there have been instances of blades with diamond-like cross-sections. This increases the weight of the sword, but makes it less prone to lateral bending during battle. Other varying factors are the tempering technique, blade curvature and tang style, each dependent on the individual school of swordcraft. Some blades also have fullers, grooves (known as hi), that ran the length of the blade. This reduces the weight with little loss to structural integrity.
Hira-zukuri blades are another straight-bladed variant of Japanese swordcraft, although in the typically muddy waters of ancient swordmaking some were also curved by heat treating. Although sometimes used to make katana, this bladeform was better suited to the shorter tanto and wakizashi knife and shortsword. Hira blades resembled chokuto, but were less 'high-tech', usually very simple and unadorned. Distinctively, the hira form has no ridges on the blade at all, and is rarely differentially tempered. A shorter sword simply does not need as much strength or careful treatment as a longer one, due to the lower maximum moment of force at it's tip. The blade of a hira sword runs parallel to the handle, and once again the tip is a graceful curve up to the spine. Almost all historic wakizashi and tanto follow this bladeform.
An ancient and rare bladeform, the kogarasu style is definitely among the strangest blades you are likely to see at a show. Only three historial examples exist in the world today, one of which is housed at the Tokyo Museum. Kogarasu blades follow the curvature of shinogi katana, but there the similarities end. Whilst the bottom half of the blade is single-edged, halfway up the blade suddenly sprouts another edge on the opposite side. The two edges meet at a tapered point reminiscent of European broadswords. In addition, a single hi runs the entire length of the blade, but in the exact centre rather than one-third from the spine. It has become a topic of academic discussion as to why this bladeform was discontinued. It is a perfectly servicable and deadly blade. It is theorised that where the single-edge becomes a double-edge, a stress point may develop over time resulting in the snapping of the blade at it's midpoint.
The kogarasu-zukuri also has a relative, the kogarasu-maru or "little crow". This blade follows the precise same geometric design, but on a larger scale, and was mounted on naginata pole-arms for use against yari spearmen. One blade of this form survives, and dates back to the early Heian period, 8th/9th century AD.
This is a true example of a double-edged katana. Moroha blades are far less common due to their impracticality and lesser strength, but examples do exist. Like the common shinogi style, the blade generally curves backwards, away from the primary striking edge. It is common for at least one of the edges to be heat-treated, and sometimes both in the case of a very skilled swordsmith. Examples exist where hamon are visible on both edges of the blade. Moroha blades usually have a ridgeline that runs the entire length of the blade, in the centre between the two cutting edges. This gives the blade a more diamond-shaped cross section. The tip is tapered far more gently than the kogarasu style, forming a long, wickedly sharp point more like a scimitar than a broadsword. It is this point that causes the greatest weakness in this bladeform, as it is generally too weak and too sharp. The weakness causes snapping in combat, whilst the sharpness causes damage to the scabbard when in storage.
The final common bladeform is the shobu style, which is essentially a mix of the shinogi and Moroha styles. Whilst single-edged, a shobu blade is shaped much like the double-edgedmoroha style, curving gracefully backwards and almost always heat-treated. The point of a shobu blade differs from more common katana by the lack of a perpendicular ridgeline; the tip sword simply follows the same curvature all the way until the edge meets the spine. This forms the long, graceful cutting point seen on moroha blades, which is generally so sharp it can be used as a cutting point rather than a piercing one. This allows the sword to be used more like a scalpel than a slashing weapon, although the same tip weaknesses are present. Shobu-styled swords generally sport a ridgeline running the length of the blade, for strength, and occasionally have hi along that line.
These bladeforms comprise the core of Japanese swordcraft. Although a lot more forms exist in varying popularities and effectiveness, this writeup can be used as a very general guide to recognising swords. The lack of diagrams here does however impair recognition somewhat, so I advise you to research the subject yourself more fully.
A special mention must go to another, very strange weapon with which I hold a personal fascination. It is not included in the above list because it is barely classified as a sword. It is known as a Nagamaki, and is an extremely heavy infantry weapon, a crossbreed of katana and naginata pole-arm. A nagamaki looks like a katana, with a very heavy blade and a handle that is the same length as the blade. Imagine for a moment how strange this looks. If you see one of these at a sword show, stop for a moment to admire it, because you will probably not see one for a long time unless you buy it. These weapons were traditionally used against cavalry and heavy infantrymen, but very few men were strong enough to wield them effectively.