baseline
The line along the coast from which a country's maritime zones are measured.
The baseline is the line the width of every maritime zone is counted from. It normally follows the low-water mark, and it closes bays, estuaries and harbors with straight segments rather than tracing them.
Water on the landward side of the baseline is internal water and is treated as part of the country itself, which is why a large bay can lie entirely inside a state while sitting outside its land.
The rules are set by the United Nations Convention on the Law of the Sea, articles 5 to 14. A country declares its own baseline within them, so the line is a legal instrument and not a measurement of the shore.
It applies to countries. A region inside a country has none of its own.
