Muromachi Koto Sword Morphology: The Transition to Uchigatana, Changing Width Proportions, and the Bo-hi

Before and After Onin: Muromachi's Morphological Divide

Muromachi sword morphology splits at the Onin War (1467): early Muromachi continues the Nanbokucho contraction trend with tachi dominant (motohaba 2.8–3.2 cm, chu-kissaki, naka-zori 1.5–2.0 cm); late Muromachi sees rapid uchigatana adoption, rewiring blade geometry around a new carrying position (blade-up, thrust through the belt).

Uchigatana Morphology vs. Tachi

Uchigatana optimized for wearing blade-up shows: shorter blade (60–73 cm), shallower sori (0.8–1.8 cm), and more uniform sakihaba/motohaba ratios (70–80%) compared to tachi. The cross-section shifts toward higher shinogi-suji ridgeline — the "shinogi-daka" form — which improves cutting force transmission and reduces weight compared to the fuller-bodied tachi cross-section.

The Bo-hi Groove: Function and Form

The bo-hi (single straight groove in the shinogiji) is the signature Muromachi addition. Its functions: weight reduction (significant in larger pieces), structural reinforcement (rib effect), and the "tama-oto" whistling sound on a swing. Late Muromachi variations include soe-hi (secondary groove alongside) and futasuji-hi (parallel double groove). Bo-hi presence or absence, and groove type, are useful dating indicators.

Bizen Osafune Mass Production

Late Muromachi Sengoku-period Bizen Osafune kazuuchi (quantity swords) show diagnostic simplifications: gunome or suguha hamon (easier to produce than choji), coarser itame hada from abbreviated folding processes, and standardized, less individualized profiles. Distinguishing these from quality Osafune pieces (Nagayoshi, Kanemitsu lineage) requires reading hada quality, hamon complexity, and nakago workmanship together.