South China Sea's vague 9-dash line underpins China's claim

Aerial view of Taiwan-controlled Taiping island, also known as Itu Aba, is seen in the Spratly archipelago on March 23, 2016. (Johnson Lai / AP)

China's South China Sea territorial claims are demarcated in vague terms by the nine-dash line, a U-shaped boundary that loops down from Taiwan as far as Indonesia's Natuna Islands.

Although China's claim to the area of about 3.5 million square kilometres (1.4 million square miles) is based on historical records and geographic proximity, the nine-dash line is a modern creation. It first appeared on 1947 maps produced by the government of the Republic of China, which was replaced by the communist People's Republic on the mainland just two years later.

While it has tried to stay out of the ongoing territorial disputes, Taiwan inherited the Republic of China's claim when Chiang Kai-shek moved his government to the island ahead of the communist advance. The Taiwanese and Chinese claims now overlap, although China has been seeking to expand its land holdings by creating new islands from reefs and atolls.

Taiwan's biggest territorial holding is Taiping island, lying roughly 2,000 kilometres (1,200 miles) south of Taiwan, where it operates a hospital, airstrip and facilities for roughly 200 residents, mostly coast guard troops.

China has refused requests to clarify the exact geographical co-ordinates of its South China Sea boundary. It also declines to say whether it considers the South China Sea to be Chinese territorial waters, an exclusive economic zone or some other legal designation.

Beijing says that in no way impedes freedom of navigation, and its diplomats accuse others of picking fights. However, rising tensions, including Indonesia's worries over China's designs on waters around the Natuna Islands, threaten to make China's preference for ambiguity no longer tenable.