sandwiching a core of hard steel between two plates of softer steel. The central plate protrudes slightly from its surrounding pieces, allowing for a sharp edge, while the softer spine protects the brittle core. Some blades had wumei or five plate construction, with two more soft plates being used at the central ridge. Bronze jian were often made in a somewhat similar manner: in this case an alloy with a high copper content would be used to make a resilient core and spine, while the edge would be made from a high-tin-content alloy for sharpness and welded onto the rest of the blade. The swordsmiths of China are often credited with the forging technology that was carried to Korea and Japan, allowing swordsmiths in those places to create such weapons as the katana. This technology included folding, inserting alloys, and differential hardening of the edge, which historically has been the most common technique around the world. While the Japanese would be more influenced by the Chinese dāo (single-edged swords of various forms), the early Japanese swords known as ken are often based on the jian. One-sided jians from the Tang dynasty provided the basis for various Japanese forging styles and techniques. The Korean version of the jian is known as the geom or gum, and these swords often preserve features found in Ming-era jian, such as openwork pommels and sharply angled tips. Korean Korea has a history of swordsmithing dating back 3,000 years. Although Korea was in close proximity to both Japan and China, no native systems of swordsmanship and swordmaking developed in Korea. Korean swords include long swords such as the yeoh do, geom, and hyup do and curved swords such as Samindo. Metal swords of double bladed leaf structure have been found throughout Korea dating back to the Bronze Age. These bronze swords were around 32 cm in overall length, with a short handle. Japanese The technology that led to the development of the Japanese sword originated in China and was brought to Japan by way of Korea. The oldest steel swords found in Japan date to the fourth or fifth century A.D. Although appearing to be ceremonial in nature, samples of these straight blades preserved in the Shōsōin were hand-forged with hardened cutting edges. By the time of the Heian period (794—1185 AD) the Japanese sword took on its distinctive curved shape as a mounted horseman would