The Ko- school arose in the district of Bingo Province during the late period and flourished through the era, producing swords of pronounced character in a region geographically and institutionally connected to the cultural heartland. Because Bingo Province contained numerous shoen estates belonging to major temples and shrines of the Kinai -- among them Toji, Rengeo-in (Sanjusangen-do), and Koyasan -- frequent interchange with Yamato is surmised, and the Yamato temperament discernible in Ko- workmanship is understood as a natural consequence of this proximity. Masaie is traditionally regarded as the school's founder, with his son Masahiro standing alongside him as one of its two foremost representatives. The lineage extends through the period and into the period, with successive generations bearing the Masaie name attested by dated works from the Bunnawa, Enbun, and Teiji eras. A related but distinct branch, the Hokke Ichijo group of Ashida District, traces its founding to Sukekuni and developed in parallel, sharing the broader Yamato-inflected temperament while maintaining its own identity.
The collective forging vocabulary of the Ko- school is anchored in mixed with and , characteristically displaying a tendency in which the grain stands prominently. Fine adheres, delicate enters the ground, and a distinctive -- a faint, whitish shadow effect -- appears across the surface, sometimes tending toward in works showing -school influence from neighbouring Province. The is fundamentally , tempered with a that consistently tends toward -- a tightened, slightly subdued quality -- accompanied by , frequent along the , and fine and running through the tempered area. The typically turns back in with in a restrained manner. A notable feature shared across the school's leading smiths is the occurrence of and even sanjuba effects, in which the temper appears doubled or tripled, producing a layered visual complexity within the otherwise calm foundation. The Shinkan Hiden-sho observes that certain Ko- works present "an appearance resembling ," confirming an -like dimension -- , , and a somewhat dark steel tone -- that enriches the predominantly Yamato character. Masaie is distinguished by numerous examples of bold construction with large , high , and broad , while Masahiro's works more often display standard proportions with ; both, however, share the school's fundamental technical identity. The Hokke branch, represented by Sukekuni and Kaneyasu, introduces a broader stylistic range encompassing with -inflected elements alongside the Yamato foundation, yet the steel texture, subdued , and remain diagnostic of their Bingo Province origin.
The Ko- school occupies a distinctive position within the broader tradition as the foremost expression of Yamato-derived swordsmithing outside Yamato Province itself. The consistently characterizes the school's output as manifesting Ko- characteristics "with particular clarity" and as possessing a quality of workmanship that is "particularly superior" within the tradition. Signed works by the school's leading figures -- Masaie and Masahiro alike -- are described as "exceedingly rare," lending each surviving inscribed example considerable documentary significance. Among the most important extant works is an exceptionally rare signed by Masaie of 101.7 cm preserved in condition at Yasukuni Shrine, while blades transmitted through the Ii family of Hikone and the Maeda house of attest to the esteem in which Ko- production has been held across centuries of Japanese connoisseurship. The school's capacity to sustain a coherent artistic identity -- defined by restrained , luminous , and subtly complex activity -- across multiple generations and related branches confirms its place as one of the most refined provincial traditions of the late and periods.