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britannica.com
"Great Wall of China".Encyclopædia Britannica. "Large parts of the fortification system date from the 7th through the 4th century BC. In the 3rd century BC Shihuangdi (Qin Shi Huang), the first emperor of a united China (under the Qin dynasty), connected a number of existing defensive walls into a single system. Traditionally, the eastern terminus of the wall was considered to be Shanhai Pass (Shanhaiguan) on the coast of the Bohai (Gulf of Zhili), and the wall's length—without its branches and other secondary sections—was thought to extend for some 6,690 km (4,160 mi)."
