Land boundaries stop at the shore, but countries also need rules at sea: who may fish, drill and sail where. Since 1982 most countries follow one treaty.
Key idea
Under the UN Convention on the Law of the Sea (UNCLOS, adopted 1982), distances are measured from the coast's baseline in nautical miles (1 nautical mile = 1,852 meters). The territorial sea runs to 12 nautical miles; the contiguous zone to 24; the exclusive economic zone (EEZ) to 200. Beyond that are the high seas, open to all.
| Zone | Distance from the baseline | What the coastal country controls |
|---|---|---|
| Territorial sea | Up to 12 nautical miles | Nearly everything, as on land; foreign ships still have the right of innocent passage |
| Contiguous zone | Up to 24 nautical miles | Enforcement of customs, immigration, tax and health laws |
| Exclusive economic zone | Up to 200 nautical miles | Rights to fish, oil, gas and other resources; other countries may sail and fly through |
| High seas | Beyond 200 nautical miles | No single country; open to all |
Why it works
The rules balance two needs. Coastal countries want their nearby fish and seabed minerals, and the means to stop smuggling. Every country needs ships to move freely, since most world trade goes by sea. So control is strongest near the shore and weakens step by step: full sovereignty, then enforcement, then resources only.
Worked example: Which zone?
A foreign fishing boat is 150 nautical miles off a country's coast. May it fish there without permission? May a foreign cargo ship sail through?
- 150 is more than 24 and less than 200, so the spot is in the exclusive economic zone.
- The EEZ gives the coastal country the rights to its fish, so the boat needs permission to fish.
- Ships of every country may sail through an EEZ.
Answer: No to fishing without permission; yes to sailing through.
Worked example: When zones overlap
Two countries face each other across a sea 300 nautical miles wide. How far can each EEZ reach?
- Each could claim 200 nautical miles, but 200 + 200 = 400, more than 300.
- The zones overlap, so the two countries must agree on a dividing line; a common choice is the line halfway between the coasts.
- Halfway: 300 ÷ 2 = 150 nautical miles each.
Answer: About 150 nautical miles each, if they use the halfway (median) line.
Common mistake
Thinking a country owns the sea out to 200 nautical miles. Beyond 12 nautical miles, ships of every country may pass; the EEZ gives rights to resources, not full control. And in the territorial sea itself, foreign ships may pass peacefully.
Most countries are parties to UNCLOS. The United States signed an agreement on its seabed mining part in 1994 but has not ratified the treaty; it follows most of the treaty's navigation and zone rules in practice. Where a continental shelf extends far from shore, a country may claim seabed rights beyond 200 nautical miles, up to set limits.