Armor and Swords — How Tachi, Katana, and Wakizashi Were Carried and Used

What Is Kosoku—Positioning of Swords in the Armor System

Kosoku refers to the collective term for small armor pieces and weapons that samurai of the Sengoku period carried alongside the main armor set (daigusoku) when wearing full armor. Kosoku includes the following.

• Small armor pieces such as kote, suneate, and haidate
• Swords such as koshikatana, uchigatana, and wakizashi

Understanding the concept of kosoku is important for grasping how Sengoku-period samurai actually carried and used swords. In films and historical dramas, scenes of armored samurai gracefully drawing their uchigatana abound, but in reality, they often differed greatly from actual battlefields. When wearing heavy armor, there were physical constraints on drawing and using swords, so methods of carrying swords suited to battlefield use developed uniquely.

The range of armor and weapons included in kosoku varies depending on the period, school, and historical sources referenced, and was not uniformly defined. However, what is common is that they filled a role as equipment that compensated for movable parts of the body and gaps that could not be covered by the main armor itself, such as oyoroi and tosugusoku. If we understand that swords too were incorporated into this practical function of "filling gaps," it becomes clear why armored samurai relied on koshikatana rather than uchigatana.

Koshikatana—The Primary Sword While Wearing Armor

It is believed that samurai wearing armor used the koshikatana rather than the uchigatana as their main weapon on the battlefield. Koshikatana refers to relatively short swords of roughly 30–60 cm blade length, close to tanto or wakizashi in modern classification, which were hung below the waist (kusazuri) of the armor or carried wakizashi-style.

When wearing heavy oyoroi or tosugusoku, drawing a long uchigatana with a blade length of 60 cm or more was difficult because various parts of the armor restricted arm and body movement. In close combat (kumitachi), a short and easy-to-handle koshikatana was advantageous for defeating enemies, and a short-sword type koshikatana specialized for thrusting techniques was practical for targeting armor gaps—the underarm, neck, and head area.

The koshikatana often employed a form called aiguchi koshirae, which lacked a tsuba. Without a tsuba, the scabbard and hilt appeared as one piece, took up less space during transport, and had practical advantages of not catching on the enemy's armor or one's own armor during close combat.

One type of koshikatana, the yoroi dooshi, was a short, slender sword specialized in piercing enemies through armor gaps. It had either a double-edged or single-edged blade and was constructed more for thrusting than cutting, with a hilt designed for a comfortable grip even when wearing kote. The yoroi dooshi was sometimes carried as a mete zashi on the opposite side from where one normally wore a sword, and it is thought that by having a backup blade positioned opposite to the dominant hand when pinning down an opponent, one could respond with a decisive thrust without losing one's stance.

The Role of Uchigatana in Battle—Close Combat and Open Battlefields

The uchigatana is a sword carried in its scabbard at the waist, with a blade length generally around 60–75 cm. It is shorter than the tachi and longer than the wakizashi.

The tachi and uchigatana differed fundamentally in the manner in which they were worn at the waist.

By wearing the uchigatana with the blade upward, drawing the scabbard and simultaneously transitioning to an upward slash became easier, making it suitable for situations requiring quick draws. These differences between tachi and uchigatana are connected to the historical development of each as a sword suited to such scenes.

In the Sengoku period, the uchigatana continued to be used, but was more often employed by lightly-armed ashigaru on foot or unarmored samurai in open battlefields, rather than by heavily-armored samurai actively engaging in combat. When not wearing armor, the uchigatana's mobile draws and successive slashing attacks functioned effectively.

From the late Muromachi through the Sengoku period, collective pike-wielding tactics developed, and combat became centered on dense pike formations rather than individual swordsmanship. This shift in battlefield conditions also meant that the uchigatana's role in warfare often remained secondary to bows, spears, and firearms.

Wakizashi—The Development of the Paired Swords and Meaning Beyond Battle

The wakizashi is a shorter sword (generally 30–60 cm blade length) carried in combination with the uchigatana. In the Edo period, the paired wearing of wakizashi and uchigatana, called daisho, was institutionalized as proper attire for samurai. However, in the Sengoku period, the concept of the wakizashi had not yet been firmly established, and diverse short swords were used interchangeably depending on circumstances.

The practice of carrying the uchigatana and wakizashi as a pair later became known as daisho and became the most fundamental dress code for samurai in the Edo period, representing their status. The fact that carrying swords was subject to certain restrictions depending on social rank, and that the act of bearing swords was bound up with status and privilege, indicates that carrying the wakizashi held meaning beyond mere practicality.

With the Edo period, the carrying of swords by the samurai class was codified into the pairing of the long sword (uchigatana) and wakizashi, reflecting political intent in weapons management. By making samurai status visible through sword-wearing style and establishing a certain level of armament socially, the shogunate sought to maintain order.

From Battlefield to Edo Streets—Historical Transformation of Carrying Methods

The transition from the Sengoku to the Edo period brought major changes to how samurai carried swords. The shift from battlefield sword-carrying centered on koshikatana adapted to heavy armor, to the daisho style as formal wear in peaceful urban society, was not merely a change in sword form but symbolized a fundamental transformation of what it meant to be samurai.

In this transition, expectations for sword fittings and maintenance evolved from those suited to a tool subjected to intense battlefield use, to those capable of withstanding appreciation and formal presentation. The sword transformed socially from "a tool for killing" to "the soul of samurai and proof of status," and the method of carrying reflected this change. When considering the Japanese sword in modern times, keeping in mind this duality where practical utility as a weapon and symbolic significance as a social marker coexist leads to a deeper understanding of Japanese sword culture.

What Existing Koshikatana, Uchigatana, and Wakizashi Reveal

Currently, most koshikatana, uchigatana, and wakizashi bearing NBTHK certificates were fitted with new koshirae and handled as art objects and heirlooms from the Edo period onward. The original state at the time of creation has not necessarily been preserved, and examples of habaki, scabbards, and hilts being remade in later years are not uncommon.

Beyond the quality of the sword blade itself, jigane, and hamon, understanding the changes in koshirae becomes a perspective for appreciating swords from the era when they were used as kosoku. The classification of koshikatana, uchigatana, and wakizashi reflects not merely differences in length, but the role each played within the armor system, and knowing this background allows one to understand both the practical and formal aspects of an individual sword more deeply.