The Great Wall is many walls

Encyclopaedia Britannica says the Great Wall actually consists of numerous walls — many of them parallel to each other — built over some two millennia across northern China and southern Mongolia. [Britannica]

UNESCO’s Statement of Outstanding Universal Value gives the Great Wall a total length of more than 20,000 kilometres, built from the 3rd century BC to the 17th century AD as the military defence project of successive Chinese empires. [UNESCO] Britannica’s “Top Questions” answer gives the total length of all sections ever built as about 21,196 kilometres (13,171 miles), including overlapping sections that were rebuilt, and the Ming wall as about 8,850 kilometres (5,499 miles). [Britannica]

UNESCO’s official short description says that in about 220 B.C., under Qin Shi Huang, sections of earlier fortifications were joined together to form a united defence system against invasions from the north. [UNESCO] The same official text says construction continued up to the Ming dynasty (1368–1644), when the Great Wall became the world’s largest military structure. [UNESCO]

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• Many walls

  A desk for the earlier states, the Qin joining, the Han rammed earth, the Ming wall, and the 2012 survey — with a table that stays in view.

• Named stretches

  From the Old Dragon’s Head at the sea to the fortress at Jiayu Pass, with photographs of real, named places.

• Myths and checks

  Whether it can be seen from the Moon or from space, set beside what NASA and Britannica actually say — and a line that still appears in UNESCO’s official text.