Heiji Era

The Heiji Era (1159–1160), though spanning barely more than one year, occupies an exceptionally important position in the history of the Japanese sword. The Heiji Rebellion (January 19 – February 5, 1159) between the Minamoto and Taira clans became the decisive phase in power struggles between the two great houses, profoundly influencing sword culture.

### Establishment of Blade Geometry

Swords from the Heiji through early Kamakura periods established the "koshi-zori" (waist-curve) form, in which curvature strengthens dramatically from the waist toward the point. This design represents swordsmiths' synthesis of cutting efficiency in mounted combat with maneuverability in foot-soldier engagements.

### Technical Development

During this era, craftsmen continuously refined sword specifications based on combat reports from various clans. Demand intensified for more practical and durable blades, propelling the field toward the maturity of the Kotō period.