The Niō school of Suō Province traces its origins to Kiyotsuna, whose earliest extant dated work bears an inscription from Bun'ei 2 (1265). The school's name derives from competing traditions: one holds that the founder rescued the Niō guardian deities from a temple fire by cutting through chains with his own blade; another suggests the name reflects residence at Niho-no-shō within the province. The lineage extended from the Kamakura period through the Nanbokuchō and Muromachi eras, with some branches continuing into the shintō period, producing multiple generations of smiths who maintained the school's distinctive technical traditions.

Niō workmanship demonstrates pronounced Yamato influence, a connection reinforced by Suō's position as the location of Tōdai-ji estates. The school's characteristic sugata features high shinogi, deep curvature, and chū-kissaki proportions. The jigane typically shows well-forged ko-itame mixed with mokume, displaying a bright steel color with prominent whitish shirake-utsuri. The hamon adheres closely to a fine suguha pattern, occasionally incorporating ko-midare or ko-gunome elements, with a tight nioiguchi in ko-nie that exhibits urumi—a moist, soft quality in the crystalline structure. Small ashi and yō enter the temper line, with hotsure, kinsuji, and sunagashi appearing as secondary features.

As documentary material for understanding provincial interpretation of Yamato forging methods, Niō blades occupy a significant position. The school sustained technical consistency across centuries while developing a recognizable house style that distinguished its output from purely Yamato work. Extant signed examples remain rare, making authenticated pieces important reference points for evaluating the numerous mumei works attributed to the tradition based on structural and metallurgical characteristics.