Public International Law (Vol. I & II)
Article 31
A treaty shall be interpreted in good faith in accordance with the ordinary meaning to be given to its terms in their context and in the light of its object and purpose.
The context includes the text, including its preamble and annexes, together with agreements and instruments made in connection with the conclusion of the treaty. Together with the context, there shall also be taken into account subsequent agreements, subsequent practice, and any relevant rules of international law applicable in the relations between the parties.