The Niō school (二王) worked in Suō Province along the San'yōdō, the western reach of the country where heavy concentrations of temple estates, including lands of Tōdai-ji, drew the province into exchange with the Yamato heartland. The members describe the line as beginning in tradition around the Hōen era (1135 to 1141) with a Kiyozane or Kiyohira, but they agree that no securely attributable work by either survives, so Kiyotsuna is regarded in effect as the school's founder. His oldest dated piece, a signed in a written-out inscription, "Bun'ei 2, third month, Kiyotsuna" (1265), is preserved at Itsukushima Shrine and serves as the touchstone against which the first generation is read; after it the signature continues without interruption through the period and into the era, the name borne by generation after generation, with a signed "Bōshū Kuga-shō Kiyotsuna" of Kenmu 2 (1335) marking the line's passage into the . The members tie the appellation most persuasively to residence in Niho-shō within the province, one tradition deriving it instead from a first Kiyotsuna who cut a chain to free the Niō Guardian Kings from a temple fire, a pun the name carries. Later hands continue the manner, the Ōei-era Kiyonaga representative of the sue-Niō generations and a Kiyosada recorded through Kakitsu, Daiei, and Tenbun.
Across the generations the members converge on one coherent manner. The is mixed with and , the grain flowing overall and in places strongly toward so that the stands finely; minute adheres and fine enter, the steel sometimes carrying a slightly whitish tone. The temper rests on a narrow , often slender, mixed with , , and a shallow , with and attached and and moving along it; along the the members note , uchi-noke, and , and the runs straight and turns back in a small , the tip at times showing . What the members single out as the school's own individuality, the features that separate Niō from the Yamashiro and Yamato it otherwise resembles, are two paired effects: a , a whitish hazy reflection standing in the toward the , and an , a moist softly clouded quality in the where the bright line of and gives way to something more subdued. Divergences across the line are matters of degree rather than kind: the early signed work reads quiet and controlled with a brightened , the later sue-Niō hands lean toward gentle with active , and the late generations add and Niō-guardian carvings in ranma- and relief within recesses.
To read Niō work the members return to the tells. The is the elegant late- form, slender and high in the , the somewhat thick, with pronounced , a small or medium , and taka-no- file marks that confirm the Yamato connection; the is with slight . At a glance the workmanship is suggestive of Yamato proper, yet the members observe that the flowing grain is not as conspicuous as in Yamato blades and the is comparatively restrained, while the in the and the in the , taken together and not either alone, are the pairing that fixes the attribution against both the Yamato heartland and the brighter Yamashiro of ; this moist, whitish register, paired with the -leaning standing , also sets the school apart from the clearer steel and of . The best of the signed first-generation pieces are judged to express not only Kiyotsuna's own hand but the characteristic traits of the school, signed examples with prized as documentary material given how few survive. Provenance, where recorded, is distinguished: an Important Art Object now in the Sano Art Museum was held at its prewar recognition by Tokugawa Iesato, another signed descends to the Seikadō from the Ōsaka collection of Kajima Isao, and the dated touchstone remains at Itsukushima. Most of what survives is held in shrine and institutional hands rather than traded, so a signed Kiyotsuna with its file marks and old two-character intact is among the rarer things a collector of provincial work might encounter.