Kotō, literally meaning “old/early sword,” encompasses a significant period in Japanese swordmaking history, spanning from the 10th century AD to the Keichō era (1596-1615) or specifically to the year 1600. The precise dividing line between kotō and shintō, the subsequent era, has been subject to scholarly debate, with various perspectives shedding light on this transition.

One milestone in defining this division is found in the Kokon Mei Zukushi Taizen, a work compiled in the 17th century based on the sword appraisal traditions of the Takeya family. This influential text designates the eighth year of Keichō (1603) as the demarcation point, possibly due to Tokugawa Ieyasu’s appointment as shōgun, which signified a significant historical turning point. Alternatively, some experts attribute this choice to the absence of new blades added to the registry during the elapsed eight years by the time the manuscripts were compiled.

However, the historical and stylistic significance of the Keichō era as a division marker has been questioned. While the Battle of Sekigahara in 1600 and the subsequent Tokugawa victory are often cited, contemporaries did not foresee the enduring peace that would follow. Some scholars propose alternative markers, such as the establishment of the tōken-kiwamedokoro in Genna two (1616), which exclusively appraised old swords (kotō), reflecting a shift in focus towards confirming the authenticity of older blades rather than newly forged ones (shintō).

Stylistic changes and advancements in steel production also played crucial roles in distinguishing kotō from shintō swords. While the Sue-kotō era saw advancements in steel quality, particularly evident in the emergence of shintō-jigane, the traditional dividing line of the Keichō era does not entirely align with these developments. Instead, shifts in jigane composition that marked a distinct departure from traditional techniques began to manifest around the Genna (1615-1624) and Kan’ei (1624-1644) eras, providing further insights into the evolution of Japanese swordmaking.