What Is Sori? 反り — Japanese Sword Curvature Explained

Sori (反り) is the curvature of a Japanese sword blade — the depth of the arc measured as the greatest distance between the back of the blade and a straight line drawn from the tip to the base of the spine. Both the amount of curve and, crucially, where along the blade it peaks are among the most powerful clues for dating a sword and identifying its tradition. Sori is a defining element of a blade's overall shape, or sugata, and one of the first measurements a specialist takes.

For a buyer, sori is both a dating fingerprint and a functional feature: the curve is what makes a katana a drawing-and-cutting weapon rather than a thrusting one. A curvature that matches the claimed era supports authenticity; one that does not can betray a shortened blade or a false signature.

How sori is created and measured

Curvature is not filed or bent into the blade — it appears largely during the clay-tempered quench. Because the thinly-clayed edge cools and contracts differently from the thickly-clayed spine, the blade springs into its arc at the moment of hardening; the smith controls the result through the shaping and the clay work. Sori is measured in millimetres as the maximum gap between the spine and the straight chord running from the tip to the notch (munemachi) at the base.

The position of the deepest point of the curve is as important as its depth, and it gives sori its named types.

Types of sori by position

  • Koshizori (腰反り) — "waist curve," deepest near the base of the blade toward the hilt; the classic profile of Heian and early Kamakura tachi, also called Bizen-zori for its association with that tradition.
  • Toriizori (鳥居反り) — "shrine-gate curve," also called torii-zori or nakazori, deepest at the centre of the blade in a balanced arc; typical of the peaceful Shinto era.
  • Sakizori (先反り) — "tip curve," deepest toward the upper blade; common on Muromachi uchigatana worn edge-up for quick foot-combat draws.
  • Muzori / chukan-zori (無反り) — little to no curve, essentially straight; seen on tanto and some special forms.

How sori dates and places a blade

Because curvature followed fighting style, it tracks the eras closely:

  • Deep koshizori points to the early Koto period of mounted warfare, when a pronounced curve near the hilt aided cutting from horseback.
  • Shallower, tip-weighted sakizori reflects the Muromachi shift to infantry and the fast-draw uchigatana.
  • Even, centred toriizori of moderate depth is the standardised Edo-period katana profile, when swords were worn as status symbols in a peaceful age.

Cross-checking the curvature against the date on the tang — using the era converter to read nengō — is a routine authenticity test.

Sori, shortening, and value

Sori is one of the features most disturbed by alteration, which makes it a key thing for buyers to read:

  • Suriage (shortening). Cutting a blade down at the tang removes part of the base, so a blade that was originally koshizori can end up looking centred or shallow. Recognising a disturbed curve helps identify a shortened Koto blade whose original signature may be lost.
  • Consistency. The sori should agree with the era and smith of the signature; a deep koshizori tachi profile under an Edo-period mei is a contradiction that suggests gimei.
  • Function and fit. Curvature determines how a blade sits in its sugata and mounts; an altered curve can mean the blade no longer matches an old scabbard, a small but real practical concern for a collector.

Frequently asked questions

What is sori on a katana?

Sori is the curvature of the blade, measured as the greatest distance between the spine and a straight line drawn from the tip to the base of the blade. Both its depth and the position where it peaks are important for dating and identification.

How is sori measured?

Sori is measured in millimetres as the maximum gap between the back of the blade and the chord running from the tip to the notch at the base of the spine. The location of that deepest point defines the type of curvature.

What are the types of sori?

The main types by position are koshizori (deepest near the hilt), toriizori (deepest at the centre), sakizori (deepest toward the tip) and muzori (essentially straight). Each is linked to a particular era and fighting style.

Why does sori matter when buying a sword?

Curvature is a strong dating clue and an authenticity check: it should match the claimed era and smith. It is also easily disturbed by shortening, so reading the sori helps reveal altered blades and lost signatures that affect value.

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