In international law, an additional protocol, complementary protocol, optional protocol or simply protocol, in analogy with the private contracts, supposes an addendum to a treaty, maintaining the validity of the main body, but modifying it, amplifying it or complementing it in some aspects .

Some protocols, because of their importance, have become more famous than the treaty itself, such as the Kyoto Protocol on Climate Change, which is part of the United Nations Framework Convention on Climate Change the 1992 Climate Change.

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