---
title: "Baseline Definition in Maritime Law | Model Diplomat"
description: "A baseline is the legal line from which a coastal State measures its maritime zones. It's essential for understanding maritime boundaries and sovereignty."
url: "https://modeldiplomat.com/learn/glossary/baseline"
---

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Model Diplomat Research Team · Last reviewed June 22, 2026

TL;DR

- A baseline is the legally defined line along a coastal State's shore from which the breadth of its maritime zones is measured under the law of the sea.
- What is the difference between a normal baseline and a straight baseline?
- Why is the baseline important for maritime zones?
- Which case first endorsed the straight-baseline method?
- How does UNCLOS treat the baseline across a bay?

[Civil Service & Diplomatic Exams](https://modeldiplomat.com/learn/civil-services/glossary)Updated June 22, 2026

A baseline is the legally defined line along a coastal State's shore from which the breadth of its maritime zones is measured under the law of the sea.

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In the 2016 South China Sea Arbitration, the Permanent Court of Arbitration ruled that China could not draw straight baselines enclosing the Spratly Islands as a unit, since UNCLOS reserves archipelagic baselines for archipelagic States.

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