The Nio school (二王) of Suo Province traces its practical origins to Kiyotsuna, who is regarded as the group's de facto founder, though tradition ascribes the school's beginnings to either Kiyozane around the Hoen era (1135-1141) or Kiyohira around the Bun'o era (1260-1261). No secure extant works by these earlier figures survive. Among Kiyotsuna's documented works, the earliest is a at Itsukushima Shrine bearing the inscription "Bun'ei 2, third month, Kiyotsuna," followed by a dated Kenmu 2 (1335) formerly in the possession of Tanzan Shrine. The lineage continued without interruption through the period and into the era. Suo Province contained extensive temple estates belonging to institutions such as Todaiji, and the pronounced Yamato character in Nio workmanship is understood as a direct consequence of this exchange with the Yamato heartland. The name "Nio" itself most likely derives from the school's residence in Niho-sho within the province.
Nio blades are forged in with a pervasive tendency toward and , producing a texture that reveals a fundamentally Yamato temperament. The is characteristically set somewhat high, and the is often broad in proportion to the . The consistently identifies a "conspicuous whitish cast" or in the forging as a hallmark of the school, alongside the signature tendency in the -- a moist, softened quality along the temper line that the examiners describe as distinctive to this group. The is overwhelmingly -based, ranging from narrow to , often with small mixed in and entering. appears along the , run intermittently, and the bosshi is typically straight with and at the tip. Where the school departs from pure , one encounters with fine , as in the of 60, though even here the Yamato foundation remains unmistakable.
The characterizes Nio work as possessing "an unpretentious and solid feeling" and "an earnest and dignified quality" -- blades that convey a plain yet substantial dignity rooted in the Yamato tradition while maintaining features that distinguish them from purely Yamato convention. Multiple emphasize that it is precisely this quality of showing "Yamato traits yet not conforming entirely to Yamato convention" that secures the Nio attribution. The school's individuality resides in the interplay of flowing , whitish , and -tinged , features that recur across works spanning the late through periods. The consistent description of their blades as -- sound and well-preserved in both and -- attests to the durability and quality of Nio forging practice across generations.