Japanese armour is thought to have evolved from the armour used in ancient China and Korea. Cuirasses and helmets were manufactured in Japan as early as the 4th century. Tankō, worn by foot soldiers and keikō, worn by horsemen were both pre-samurai types of early Japanese cuirass constructed from iron plates connected together by leather thongs.
During the Heian period (794-1185), the Japanese cuirass evolved into the more familiar style of armour worn by the samurai known as the dou or dō. Japanese armour makers started to use leather (nerigawa) and lacquer was used to weather proof the armor parts. By the end of the Heian period the Japanese cuirass had arrived at the shape recognized as being distinctly samurai. Leather and or iron scales were used to construct samurai armours, with leather and eventually silk lace used to connect the individual scales (kozane) which these cuirasses were now being made from.
In the 16th century Japan began trading with Europe during what would become known as the Nanban trade. Samurai acquired European armour including the cuirass and comb morion which they modified and combined with domestic armour as it provided better protection from the newly introduced matchlock muskets known as Tanegashima. The introduction of the tanegashima by the Portuguese in 1543 changed the nature of warfare in Japan causing the Japanese armour makers to change the design of their armours from the centuries-old lamellar armours to plate armour constructed from iron and steel plates which was called tosei gusoku (new armours). Bullet resistant armours were developed called tameshi gusoku or (bullet tested) allowing samurai to continue wearing their armour despite the use of firearms.
The era of warfare called the Sengoku period ended around 1600, Japan was united and entered the peaceful Edo period, samurai continued to use both plate and lamellar armour as a symbol of their status but traditional armours were no longer necessary for battles. During the Edo period light weight, portable and secret hidden armours became popular as there was still a need for personal protection. Civil strife, duels, assassinations, peasant revolts required the use of armours such as the kusari katabira (chain armour jacket) and armoured sleeves as well as other types of armour which could be worn under ordinary clothing. Edo period samurai were in charge of internal security and would wear various types of kusari gusoku (chain armour) and shin and arm protection as well as forehead protectors (hachi-gane).