The Philippines also asked the tribunal to classify whether a number of disputed areas are islands, low-tide coral outcrops or submerged banks to determine the stretch of territorial waters they are entitled to under the ­convention. The convention does not deal with sovereignty questions, which the Philippine government says it did not raise.

What is the origin of the line?
It appeared on a Chinese map as an 11-dash line in 1947 as the then Republic of China’s navy took control of some islands in the South China Sea that had been ­occupied by Japan during the second world war. After the People’s Republic of China was founded in 1949 and Kuomintang forces fled to Taiwan, the communist government declared itself the sole ­legitimate representative of China and inherited all the nation’s maritime claims in the region.

But two “dashes” were removed in the early 1950s to bypass the Gulf of Tonkin as a gesture to communist comrades in North Vietnam.

Beijing intensified its hold in the northern part of the waters in the mid-1970s when it expelled the South Vietnamese navy from the Paracel Islands after a clash that saw dozens killed.

Seven out of about 200 reefs in the Spratly Islands came under Chinese control in the 1980s and 1990s and Scarborough Shoal in 2012. Taiwan still maintains its maritime claims in the region and has kept a military garrison on Pratas Islands and the largest ­natural feature in the Spratlys, ­Taiping.
Why is the line so important?
It serves as the basis of China’s claim to “historical rights” in the region, as neither Beijing nor Taipei ever held effective control over the entire region encompassing more than 2 million sq km. Other claimants such as the Philippines, Malaysia and Brunei root their claim in geographical proximity, while Vietnam, which occupies the largest number of islands and reefs in the Spratlys, at 29, stresses it actively administers the area. The Philippines is challenging the legality of the line at the international tribunal under Unclos.

What will guide China’s response to the South China Sea tribunal ruling?

According to the treaty, a nation has sovereignty over waters ­extending 12 nautical miles from its land and exclusive control over economic activities 200 nautical miles out. Beijing maintains it has historical evidence proving its control of territory further out to sea. If the Philippines wins, China’s claims would be seriously undermined and neighbouring states would have a legal endorsement for their position.

Beijing argues the nine-dash line emerged in the new world ­order after the second world war and came well before the 1982 Unclos. Beijing says China accepted the Japanese surrender and reclaimed the region with legal backing and the authorisation of the Allies. Other claimants in the region and the US expressed no objections at the time, it argues.

What is Beijing’s strategy with the nine-dash line?

Beijing is a signatory to Unclos, but it has intentionally never ­defined the legal meaning of the nine-dash line or what its “rights” are within the boundary. This ambiguity has led to the idea among many ordinary Chinese people that it marks the nation’s maritime boundary, but again, Beijing has never made this explicit.

Others say it encircles the area where China demands economic rights. Another interpretation is the line marks the islands and reefs China wants to control rather than the waters inside its boundaries. ­Beijing has long favoured a strategy of ambiguity. It does not openly go against international law, but prefers to leave space for its more ambitious claims.
What happens if the tribunal rules against China?
The consensus among legal experts is that the court is unlikely to rule specifically on the nine-dash line. The court has said earlier it will not offer a judgment on territorial disputes, but there is a small chance it may rule on whether there is a legal basis for the line under the UN convention. If it rules against China, the government may face increased international pressure to clarify its position on the line’s legal justification. But what is virtually certain is China will not remove the line from its maps, especially given growing nationalism.

Beijing has also repeatedly said it will ignore any rulings by the tribunal. Taiwan has said it stands by its position that all South China Sea islands are its territory. The island’s leader, Tsai Ing-wen, has not mentioned the nine or 11-dash line and has emphasised it will adhere to international law. If she were to give up the nine-dash line claims following the ruling, cross-strait relations would likely be further strained.

Additional reporting by Associated Press