Understanding katana geometry — the precise vocabulary for describing a blade’s shape, curvature, cross-section, and proportions — is essential for collectors, practitioners, and anyone reading nihonto scholarship. A sword described in Japanese classification terms conveys information that “curved Japanese sword” cannot.
Blade Length: Nagasa
The nagasa is the blade length measured from the back of the machi (the notch at the base of the cutting edge where blade meets tang) to the tip. Standard katana nagasa is 60–73 cm. Tachi tend to be longer (above 60 cm), while wakizashi are 30–60 cm and tanto under 30 cm. Individual smiths had preferred lengths, and the nagasa of historical blades was sometimes adjusted (shortened from the base) in later periods to repair damage or change the sword’s balance.
Curvature: Sori
Sori describes the degree of curvature of the blade. It is measured as the maximum perpendicular distance between the back of the blade and a straight line connecting the machi and the tip. Standard katana sori is approximately 1.5–2.0 cm. Koshi-zori (hip curvature) describes curvature concentrated toward the base — common in early tachi. Sakizori describes curvature concentrated toward the tip — common in late Sengoku and Edo period blades suited to thrusting. Tori-zori is even curvature throughout the blade, the most common form in katana.
Cross-Section: Hira-niku and Shinogi
The blade cross-section defines cutting performance and visual character. The shinogi is a longitudinal ridge running along the upper portion of the blade — the transition between the bevel leading to the edge and the flat area below the spine. The width and height of the shinogi affects how the blade cuts: a high shinogi produces a more rigid blade; a lower shinogi increases the flat area and changes the cutting geometry.
The hira-niku is the fullness of the blade surface between the edge and the shinogi. A blade with good hira-niku has convex surface geometry (hamaguri-ha, or “clam-shell edge”) rather than flat or hollow-ground. Convex geometry is more durable against edge chipping and is the traditional form; it requires more work to sharpen but performs better in cutting through material.
Tip: Kissaki
The kissaki is the tip section of the blade, separated from the main body by a curved line called the yokote. Kissaki size and shape is a significant indicator of period and school. Ko-kissaki (small tip) is associated with early Koto blades and the refined Yamashiro tradition. Chu-kissaki (medium tip) is the standard for most katana periods. O-kissaki (large tip) appears on Nanbokucho-period blades designed for the powerful two-handed cuts of mass infantry combat, and revived in the Shinshinto period as an aesthetic reference.
Width and Taper
Motohaba (base width) and sakihaba (tip width) together describe the blade’s taper. The ratio between them (sakihaba/motohaba) reflects the period and school: Koto blades typically show significant taper; Shinto blades tend toward more parallel sides. Kasane (blade thickness) at the base versus tip similarly reflects period — thin-spined blades are associated with the refined aesthetic of Yamashiro; thick spines with the powerful combat focus of Soshu and Mino.