Sword Morphology of Early Heian Japan: Koshi-zori, Slender Body, and Ko-kissaki

Reading Japanese Sword Morphology

Sword scholars analyze five key morphological parameters: sori (curvature amount and position), mihaba (blade width at base vs. tip), kasane (blade thickness), kissaki geometry, and the type of sori (koshi-zori/naka-zori/saki-zori). Early Heian period (794–late 10th century) swords — the formative era of tachi development — exhibit distinctive morphological signatures unlike later periods.

Koshi-zori: Base-Weighted Curvature

The most distinctive early Heian characteristic is koshi-zori: curvature peaking in the lower third of the blade near the nakago. Measured examples show maximum curvature at 30–40% of blade length from the base. This contrasts with later naka-zori (mid-blade peak) of Kamakura swords. Koshi-zori facilitated drawing the tachi (worn blade-down, suspended) from horseback without interference with the saddle.

Slender Width Ratio and Strong Taper

Early Heian tachi show sakihaba/motohaba ratios of 55–65% — significantly more tapered than mid-Kamakura swords (65–75%). This slender, strongly tapered body reflects both continental sword aesthetic inheritance and the technical limitations of early folded-steel forging. Examples attributed to early koto production (Furu-hoki style pieces, 9th–10th century) confirm these measurements.

Ko-kissaki: The Small Point Form

The small, restrained kissaki of early Heian swords predates the inokubi-kissaki (boar-neck point) of Kamakura and the ō-kissaki of Nanbokucho. Ko-kissaki reflects both technical constraints in forming precision point geometry and a blade optimized for mounted slashing rather than thrusting. The transition to larger kissaki forms tracks the evolution of battlefield tactics and forging mastery across subsequent centuries.