Jõkotõ are straight Japanese swords with no curvature. They are usually constructed in the hira-zukuri and kiriha-zukuri styles. It is] thought that the shift from straight blades to Japanese swords with curvature happened around the mid to late Heian period (794-1184). This was during the mid 10th century: about the time Taira Masakado and Fujiwara Sumitomo rebelled against the government in the Johei (931-938) and Tengyo (938-947) eras. Blades before these are continental style blades called jõkutõ, and were brought to Japan from the Asian continent.
Examples of theseJapanese swords have been excavated from Kofun period tombs, and some still reside in the Shosoin Imperial Repository, Nara