Distinguish between treaty-based obligations and customary international law using the example of transit passage through international straits.

Article 38 of the Statute of the International Court of Justice recognises both "international conventions" and "international custom, as evidence of a general practice accepted as law" as sources of international law [1]. Though both bind states, they differ in origin, reach and proof — a distinction best illustrated by the regime of transit passage through straits used for international navigation.

Basis of obligation

• Treaty law rests on express consent — signature, ratification or accession. It binds only parties, following the rule that treaties create neither rights nor duties for third states.
• Custom arises from general and consistent state practice coupled with opinio juris (acceptance as law), and binds all states irrespective of formal assent [1].

Formation, proof and exit

• Treaties are written, dated and precise: UNCLOS was opened for signature at Montego Bay on 10 December 1982 and entered into force on 16 November 1994 [2].
• Custom is unwritten; its existence must be demonstrated from practice, making its content less certain but its coverage wider.
• A party may denounce or reserve against a treaty; escaping custom requires sustained objection from the outset.

Illustration: transit passage

• Part III of UNCLOS (Articles 37–44) grants ships and aircraft of all states the right of transit passage — continuous and expeditious passage through straits linking two parts of the high seas or EEZ — which the bordering state shall not impede or suspend [3].
• For UNCLOS parties, this is a treaty obligation.
• Yet non-parties such as the United States and Iran [4] are widely regarded as bound by the same regime as customary law, since it reflects settled navigational practice. Hence disruption of the Strait of Hormuz, a chokepoint critical to India's energy imports, attracts international legal objection despite Iran's non-membership [5].

The two sources are therefore complementary rather than rival: treaties codify and clarify, while custom universalises. UNCLOS, aptly called the "constitution for the oceans" [2], derives its authority from both — a lesson India, as a party and a major Indian Ocean stakeholder, can invoke to anchor freedom of navigation in a rules-based maritime order.