Kaiken, Koshigatana, and Yoroidoshi Compared: The Morphological Differences Between Tanto-Adjacent Blades and Their Roles in Samurai Culture

"The Complex World of 'Swords Resembling Tanto'"

In the classification system of Japanese swords, it is customary to define tanto as blades with a blade length under one shaku (approximately 30cm). However, in actual samurai society, there existed a diverse range of short-bladed swords that did not fit neatly into this definition. Kaiken, koshigatana, and yoroidoshi are all easily confused with tanto, but each possesses distinct characteristics in form, purpose, and social significance.

Organizing these distinctions is also important in the collection and appraisal of Japanese swords. The designation of "type" in certificates of authenticity, classification in auctions and specialized publications—in practical contexts, the differences among these three types influence judgment.

Kaiken (懐剣)——Personal Defense Blade and Ritual Significance of Samurai Women

Kaiken were small blades carried by samurai women in their bosom or within their sash. The blade length was generally three to five sun (9–15cm), even shorter than the lower end of tanto.

The most distinctive feature of kaiken is the form of the mounting (koshirae). Commonly referred to as "aiguchi construction," they had a smooth form without a tsuba, and were carried sheathed in a silk bag (kaiken bag). The blade was often forged in hira-zukuri, with a thin and flat cross-section, designed so that its shape would not be visible even when concealed within clothing.

Purpose divided into primarily two layers: personal defense and ritual. For defense purposes, they are said to have served as a means of ritual suicide in the worst circumstances, and at samurai weddings there was a custom of brides bringing a "keepsake kaiken." For ritual purposes, they were sometimes given to samurai sons and daughters as proof of adulthood.

In modern times, kaiken is widely recognized as "the blade of samurai women," though there are historical records of male samurai also carrying them for personal protection. Additionally, since kaiken fall under the scope of the Swords and Firearms Control Law, possession of antique kaiken in modern times requires a registration certificate.

Koshigatana (腰刀)——Versatile Short Blade Between Utility and Decoration

Koshigatana broadly refers to "all small blades worn at the waist," but in the narrower sense refers to a short-bladed sword with a specific form. The blade length is generally four to eight sun (12–24cm), and often positions itself between tanto and kaiken.

The characteristic of koshigatana is the diversity of its forms. Various formats exist, with or without a tsuba, in hira-zukuri or shinogi-zukuri styles, making it difficult to define comprehensively as "if this, then it's koshigatana." Rather, it is more appropriate to understand it as a classification based on how it was used and its purpose, with the functional definition of "a small blade worn at the waist and used in daily life" coming closer to reality.

From the Heian to Muromachi periods, there are records of high-quality koshigatana forged by smiths being used as gifts to the Imperial Court and powerful samurai clans. Koshigatana with intricate designs in the mountings (hilt, sheath, fittings) also possessed an aspect as works of art that prioritized decoration over practicality.

From the Edo period onward, as more distinct classifications such as razors, wakizashi, and tanto became established, the term "koshigatana" itself became old-fashioned. In the modern sword market, the classification of koshigatana is sometimes used for pre-Edo works.

Yoroidoshi (鎧通し)——Distinctive Form of a Battlefield Practical Tool

Yoroidoshi is, as the name suggests, a short-bladed sword designed for the purpose of "piercing through the gaps in armor." The blade length is approximately three to six sun (9–18cm), and it possesses a distinctive cross-sectional form that embodies its name.

Whereas ordinary tanto are made in hira-zukuri or shinogi-zukuri, yoroidoshi are often made with a triangular cross-section (sanryou-zukuri) or square cross-section (kaku-zukuri). This form was optimized for the motion of "pushing through" rather than "cutting with the blade" when inserting it through the small openings and joints of armor.

It is also characteristic that many examples lack a hamon (or have an inconspicuous one), which is said to be because the design of the hardening process through quenching prioritized the rigidity of the entire blade over the edge. The character of yoroidoshi as a practical battlefield tool—used by armored cavalry to strike down opponents whether mounted or on foot—is clearly reflected in its form.

With the end of the Sengoku period, the demand for yoroidoshi in actual combat decreased sharply. There are few examples from the Edo period onward, and most surviving yoroidoshi are antiques from the Muromachi to Sengoku periods. For this reason, in the modern sword market they command high rarity value, and well-preserved antique yoroidoshi often receive high evaluation.

Comparative Morphology of the Three Types——Identification Points in Appraisal

The following observation points are effective when identifying these three types in actual appraisal settings.

Blade length and proportion: Kaiken are three to five sun with koshirae that are invariably aiguchi construction. Koshigatana are four to eight sun with diverse formats. Yoroidoshi are three to six sun with distinctive cross-sectional form.

Cross-sectional form: The triangular and square cross-sections of yoroidoshi can be clearly distinguished from the other two types. Kaiken and koshigatana are often in hira-zukuri, but there are cases where the cross-section alone cannot determine the distinction.

Form and signature of the nakago: Blades forged exclusively as yoroidoshi sometimes have a signature explicitly stating "yoroidoshi" on the tang. Additionally, koshigatana from the Heian to Muromachi periods show era-specific characteristics in the form of the tang and the file marks.

Form of the koshirae: Kaiken are invariably aiguchi construction. Yoroidoshi often feature koshirae without a tsuba and with a slender profile. Koshigatana have the most diverse formats, ranging from elaborate metalwork koshirae to simple wooden sheaths.

Division of Use in Samurai Culture——How They Were "Differentiated"

When organizing how these three types were differentiated in samurai society, a division based on hierarchy, gender, and circumstance becomes apparent.

Kaiken belonged primarily to the realm of women and ritual, serving dual purposes of protection and ceremony. Koshigatana possessed two faces: practical daily use and gifts, and particularly during the Heian to Muromachi periods, differences in quality proportional to social rank were reflected in surviving examples. Yoroidoshi were specialized weapons for the extreme circumstances of the battlefield, and their demand vanished in the peaceful Edo period.

For modern sword enthusiasts, understanding the differences among these three types provides a perspective that does not lump swords together as mere "short blades," but rather understands each through its own historical context and use circumstances. The comparison of these three types clearly demonstrates that the diversity of Japanese swords is inseparable from the diversity of their social contexts.