In the late Heian period, especially around the time of Early Nine-Years War and Late Three-Years War, Tachi was developed along with increasing power of samurai, and usually the ones after this period are called Japanese swords.
平安時代後期、特に武家勢力が活発になった前九年の役や後三年の役あたりから武家の勢力が増大し、これに伴い太刀が発達し、通常これ以降の物を日本刀とする。