How Are Lines Drawn at Sea, and Who Decides When Claims Overlap?

How is the sea divided, moving out from the coast?

Everything is measured from the baseline. Normally that is the low-water line along the coast; where the coastline is deeply indented, or fringed with islands close offshore, a state may instead draw straight lines between selected points. Waters on the landward side are internal waters. Seaward of the line, the 1982 UN Convention on the Law of the Sea (UNCLOS) divides the sea into bands by distance, each carrying different rights.

The first 12 nautical miles (about 22 km) form the territorial sea. The coastal state’s sovereignty covers the water, the airspace above it and the seabed and subsoil below. Foreign ships enjoy a right of “innocent passage”: they may pass through, but fishing, surveying or exercising with weapons on the way means the passage is no longer innocent. Out to 24 miles lies the contiguous zone, where the coastal state may exercise the control needed to prevent or punish breaches of its customs, fiscal, immigration and sanitary laws within its territory or territorial sea.

Out to 200 miles (about 370 km) runs the exclusive economic zone. The seabed and subsoil beneath form the continental shelf, which normally also extends to 200 miles. Where the continental margin reaches further, the shelf can follow it, subject to review by the Commission on the Limits of the Continental Shelf and to a cap of 350 miles, or 100 miles beyond the 2,500-metre depth contour. Beyond the exclusive economic zone lie the high seas, over which no state may claim sovereignty.

Every one of these distances starts from land. Without land there is no baseline, and without a baseline there is no maritime zone, which is why disputes at sea so often come back to the shore: who owns a given piece of land, and whether it counts as a starting point at all.

Does a country own its exclusive economic zone?

Not the way it owns its territory. What the coastal state holds there are “sovereign rights”, and the convention spells them out: exploring, exploiting, conserving and managing the natural resources of the water, the seabed and the subsoil, from fish and hydrocarbons to energy from water, currents and wind. It also has jurisdiction over artificial islands and installations, marine scientific research and the protection of the marine environment.

Other states’ rights are written down as well. All of them enjoy freedom of navigation and overflight in another country’s zone, along with the freedom to lay submarine cables and pipelines. Each side must show “due regard” for the other: the coastal state cannot use its resource rights to block passage, and foreign ships must comply with the laws it makes in line with the convention.

Enforcement needs a legal basis too. The convention, for instance, expressly allows a coastal state to board, inspect and arrest vessels in its exclusive economic zone that break its laws on fishing and other living resources. Sailing through, by contrast, is every state’s freedom and needs no one’s permission.

Since June 2026 China has repeatedly sent ships east of Taiwan to “enforce the law”, presenting this as a response to the Japan–Philippines boundary talks. Taiwan’s coast guard says that during the June operation Chinese coast guard ships questioned merchant vessels about their ports of origin and destination and claimed jurisdiction; on a patrol in July, two of them were 54 nautical miles east of Hualien, far outside the territorial sea and the contiguous zone. Sailing there is unremarkable in itself. The dispute is over the claim to jurisdiction, and presenting such a claim as routine is how a grey-zone campaign works at sea.

When two countries’ zones overlap, who decides?

When two coasts are less than 400 nautical miles apart, their 200-mile zones overlap. For exclusive economic zones and continental shelves the convention offers a principle rather than a ready-made line: the boundary is to be fixed by agreement on the basis of international law, in order to achieve an equitable solution (Articles 74 and 83). Overlapping territorial seas default to a median line, unless the two states agree otherwise or historic title or other special circumstances call for something different (Article 15).

If talks stall, the convention makes two provisions. States that cannot agree within a reasonable time are to turn to its dispute settlement procedures, although a state may declare in advance that boundary disputes are excluded from compulsory arbitration and similar procedures, as China did in 2006. And until a deal is reached, the two sides are to make every effort to enter into “provisional arrangements of a practical nature”, which do not prejudice the final boundary.

Any agreement binds only the states that sign it. After Japan and the Philippines announced in May 2026 that they would open talks on their exclusive economic zone and continental shelf boundary, Japan’s chief cabinet secretary, Minoru Kihara, said an agreement would not be legally binding on third parties, and Taiwan’s foreign ministry cited the Vienna Convention on the Law of Treaties to the same effect. As of June 2026 the two governments had neither formally begun talks nor set a timetable.

Why does it matter so much whether a speck of land is an island or a rock?

The zones are measured from land, and the convention ranks land. Under Article 121, an island is a naturally formed area of land, surrounded by water, that stays above water at high tide, and it generates the same zones as any other territory. But “rocks which cannot sustain human habitation or economic life of their own” get no exclusive economic zone or continental shelf, only a territorial sea and a contiguous zone.

The arithmetic shows the stakes. An isolated feature in open ocean that counts as an island can claim a circle of sea 200 miles in radius, roughly 430,000 square kilometres; if it counts as a rock, its 12-mile territorial sea covers about 1,550 square kilometres, less than a two-hundredth of that. The tribunal in the 2016 South China Sea arbitration said the rule was meant to stop insignificant features from generating large zones at the expense of inhabited territory and the high seas.

Below rocks come low-tide elevations, which surface at low tide and are covered at high tide; one lying more than 12 miles from land has no territorial sea of its own (Article 13). Artificial islands and installations do not count as islands at all: they have no territorial sea and do not affect boundaries (Article 60). A feature’s status depends on its natural condition, so reclaiming land or building on a reef does not change what it is.

Because the sea follows the land, ownership of the land itself becomes worth contesting. In June 2026 a seminar at China’s Jinan University claimed that the Batanes Islands legally belong to what China calls “Taiwan Island”. Our analysis of the Batanes claim argues that it was likely an attempt to weaken the legal basis of the maritime claims the Philippines makes from the islands, since whoever holds an island holds the starting point.

What did the 2016 South China Sea ruling actually decide?

In January 2013 the Philippines brought a case against China under UNCLOS. It was heard by a tribunal constituted under the convention’s Annex VII, with the Permanent Court of Arbitration acting as registry. China said it would neither accept nor participate; under Annex VII, the absence of one party does not bar the proceedings. The tribunal delivered a unanimous award on 12 July 2016.

Two findings sit at its core. First, China’s claims to historic rights to resources within the “nine-dash line” have no legal basis to the extent that they exceed what the convention allows. Second, every feature in the Spratly Islands that stands above water at high tide, Itu Aba (Taiping Island) included, is legally a rock with no exclusive economic zone or continental shelf, while Mischief Reef and Second Thomas Shoal are low-tide elevations lying within the Philippines’ exclusive economic zone and continental shelf.

The test for a rock is whether a feature, in its natural condition, can sustain a stable community of people or economic activity that neither depends on outside resources nor is purely extractive. Military or government personnel kept supplied from outside do not demonstrate that capacity, the tribunal held, and the historical record showed only fishermen staying for short periods and Japanese fishing and guano-mining ventures in the 1920s and 1930s.

The tribunal stressed that it was neither ruling on sovereignty over any land nor drawing any boundary. It decided how much sea a feature can generate, not who owns it, which is why China’s 2006 declaration excluding boundary disputes did not stop the case. Under the convention the award binds only the two parties, and only in respect of that dispute. China and the Philippines have had repeated run-ins at sea in recent years, and the US Senate’s draft National Defense Authorization Act for fiscal 2027 calls for a South China Sea crisis management strategy.

What does this mean for Taiwan?

Taiwan sits where several sets of claims meet. To the east, its foreign ministry says the area Japan and the Philippines intend to delimit overlaps heavily with Taiwan’s exclusive economic zone off its east coast. It has called on both governments to take Taiwan’s rights fully into account and to consult it, and has told its offices in Tokyo and Manila to confirm that the talks will affect neither Taiwan’s sovereign rights under international law nor the operation of its 2013 fisheries agreement with Japan and its 2015 fisheries law-enforcement cooperation agreement with the Philippines.

The ministry has also pointed out that an exclusive economic zone is a set of sovereign rights and jurisdiction over marine resources, not territory or territorial sea, so the Japan–Philippines talks do not touch Taiwan’s sovereignty. The more direct pressure comes from Beijing. China’s foreign ministry called the talks “completely illegal, null and void”, and China then sent ships east of Taiwan to “enforce the law”; Taipei says China has no right to claim enforcement powers in those waters. Our analysis of the patrols east of Taiwan traces how the campaign works.

To the south, Taiwan administers Itu Aba, the largest feature in the Spratlys that stands above water at high tide: about 0.43 square kilometres, with buildings, a lighthouse, a runway and port facilities. The tribunal still classed it as a rock. Taiwan was not a party to the case. In 2016 its foreign ministry called the award completely unacceptable and without any legal force for the Republic of China, objecting among other things that it referred to Taiwan as the “Taiwan Authority of China” and that the tribunal never invited Taiwan to take part or sought its views. In July 2026 the ministry said its South China Sea position was unchanged and that Taiwan should be included in multilateral dispute settlement mechanisms.

The two cases share a pattern: decisions about the waters around Taiwan are often taken in forums where Taiwan has no seat. Under the convention and the law of treaties, neither the award nor any future Japan–Philippines agreement binds Taiwan, but both may shape how other governments see the seas around it, including the stretch east of Taiwan in the middle of the first island chain. Beyond stating its position, Taiwan can keep its existing fisheries arrangements working and decline to complete the last step of someone else’s “enforcement”: its coast guard has told ships not to respond to so-called boarding inspections.