Double taxation treaty applies to residents of one or both states and is given domestic effect under tax law. A bilateral Convention between India and Portugal for the avoidance of double taxation and prevention of fiscal evasion has been given effect in India under section 90 of the Income-tax Act. Article 1 sets the personal scope: the Convention applies to persons who are residents of one or both Contracting States, establishing residency as the primary test for entitlement to treaty provisions and related anti-evasion measures.