The Nio school (二王) of Suo Province traces its practical origins to Kiyotsuna, whose earliest surviving work -- a at Itsukushima Shrine dated Bun'ei 2 (1265) -- establishes the lineage as active from the mid- period. Although tradition ascribes the school's founding to either Kiyozane around the Hoen era (1135--1141) or Kiyohira around the Bun'o era (1260--1261), no securely attributable works by these figures survive, and Kiyotsuna is today regarded as the de facto progenitor. The appellation "Nio" most likely derives from the school's residence in Niho-sho within the province. Suo contained extensive estates belonging to major temples, beginning with Todai-, and the pronounced Yamato character pervading Nio workmanship is understood as a direct consequence of this institutional exchange with the Yamato heartland. From Kiyotsuna's generation, the lineage continued without interruption -- through figures such as Kiyonaga and Kiyosada, whose names perpetuate the school's characteristic "Kiyo" (清) leading character -- into the period and even into the era.
The collective forging identity of the Nio school is distinguished by two diagnostic traits that the identifies with remarkable consistency across the designated corpus: , a whitish, hazy reflection in the steel, and , a moist, softened quality along the temper line. The is characteristically with a pervasive tendency toward and , producing a texture that reveals a fundamentally Yamato temperament. The is set somewhat high and the is broad in proportion to the , structural features that plainly manifest the Yamato tradition's principles. Kiyotsuna's forging displays fine adhering closely with delicate , while the later Kiyonaga shows a closely packed with a slightly whitish tone. Across the school, the is overwhelmingly -based -- ranging from narrow to , with small mixed in, entering, and appearing along the . and run intermittently, and the is typically straight with and at the tip. Kiyosada's later works incorporate a gentle element with active , and Kiyonaga's bear relief carvings of Nio guardian figures within recesses -- an iconographic signature consistent with the school's name.
The Nio school occupies a position of quiet but unmistakable distinction among the provincial sword-making traditions of western Japan. The characterizes their 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 aesthetic. Multiple designation records emphasize that it is precisely the quality of showing "Yamato traits yet not conforming entirely to Yamato convention" that secures the Nio attribution: the pervading the and the -tinged constitute features not found in the home province schools themselves, marking the Nio as a lineage that absorbed the Yamato spirit through temple culture rather than direct apprenticeship. Signed examples by Kiyotsuna with preserving original taka-no- file marks are singled out as "exceedingly valuable" and "extremely precious" as reference material, while Kiyosada's in the Imperial Collection demonstrates the school's range across blade forms. The consistent description of their blades as -- sound and well-preserved in both and -- attests to the durability and integrity of Nio forging practice, a provincial lineage of contemplative refinement whose individuality resides in the interplay of flowing grain, whitish reflection, and moist temper that recurs across works spanning from the late through the period.