Ko- (古手掻) is the founding and classical phase of the school, spanning the late period through the era. The school takes its name from the gate (転害門) of Tōdai-ji in Nara, near which its smiths lived and worked under the temple's patronage, and its founder is Kanenaga (包永), traditionally dated to around the Shōō era (c.1288-1293). Kanenaga is reckoned among the foremost Yamato masters: his signed are celebrated for combining the dignity of form with the disciplined grain of the Yamato tradition, and several are designated at the highest levels. The mainline continued through a second-generation Kanenaga and the founder's sons and pupils — Kanetsugu (包次), Kanekiyo (包清), and the early Kanezane (包眞) — into the period.
In workmanship Ko- is orthodox . The is with conspicuous , especially along the , thick with and threaded by . The is built on a narrow, -laden , frequently broken by and , with and along the and abundant and ; the runs to a or finish, often continuing into . Early works show a slender, deeply with a small , broadening into the more powerful forms of the period. The school's -rich grain and quiet, -active distinguish its blades from the -based exuberance of contemporaneous work and place squarely within the five Yamato schools.