Scholars agree that Japanese armour first appeared in the 4th century, with the discovery of the cuirass and basic helmets in graves. It is thought they originated from China via Korea. During the Heian period (794-1185), the unique Japanese samurai armour ō-yoroi and dō-maru appeared. The Japanese cuirass evolved into the more familiar style of body armour worn by the samurai known as the dou or dō, with the use of leather straps (nerigawa), and lacquer for weatherproofing. Leather and/or iron scales were also used to construct samurai armours, with leather and eventually silk lace used to connect the individual scales (kozane) of these cuirasses. In the 16th century, Japan began trading with Europe, during what would become known as the Nanban trade. This was the first time matchlock muskets were imported, and as they became mass-produced domestically, samurai needed lighter and more protective armour. As a result, a new style of armour called tosei-gusoku (gusoku), which means modern armour, appeared. When a united Japan entered the peaceful Edo period, samurai continued to use both plate and lamellar armour as a symbol of their status.