The 'golden age' of the Japanese sword came about due to the rise of the warrior class, and the use of the sword as a weapon in the nationwide civil wars of AD 1180-85 and the ongoing conflicts of the Kamakura and the Muromachi (1333-1573) periods. Swords of the Heian period had been long (up to 80cm), thinning to half the width towards the point. In the Kamakura period they now became broader and more robust. The tachi, worn slung on the left side, was long and tapered in a graceful curve; the blade was sharp yet extremely strong. A shorter dagger (tantô) was also worn and used for hand-to-hand fighting. The Mongol invasions led by Kublai Khan in 1274 and 1281 brought a change in combat from cavalry to infantry. Swords became longer, sometimes 150cm along the cutting edge, and had a more even curve.

The production of a sword was a multi-stage process of folding, hammering and welding, using several grades of steel. The heat treatment used to harden the blade created various crystalline patterns (hamon) along the cutting edge. The subsequent tempering and polishing also required highly skilled, painstaking stages.

A concentration of sword-makers developed at the seat of government in Kamakura, but the smiths of Bizen province (present-day Okayama Prefecture) also continued to produce some of the finest blades. Fine swords were believed to have spiritual qualities and formed a part of the imperial regalia.