Which statement best distinguishes treaty law from customary international law in cross-border cooperation?

• A. Treaty law consists of unwritten norms; customary international law arises from consistent state practice.
• B. Treaty law consists of written agreements binding states; customary international law arises from consistent state practice accompanied by a belief it is legally obligatory.
• C. Treaty law governs internal affairs; customary law governs only maritime boundaries.
• D. Treaty law applies during war; customary law applies during peacetime.

Answer: (B)

Treaty law is about formal, written agreements between states that create binding rights and obligations for the parties who sign them. These treaties explicitly spell out how states will cooperate across borders, and they become legally enforceable through the consent of the states involved.

Customary international law, by contrast, comes from long-standing state practice that is generally accepted as legally obligatory. It isn’t written in a single document, but it develops over time as states repeatedly engage in certain actions and believe those actions are required by law (opinio juris). This kind of law fills gaps where there isn’t a treaty, guiding cross-border conduct through unwritten norms.

So the statement that best distinguishes them is that treaty law consists of written agreements binding states, while customary international law arises from consistent state practice accompanied by a belief that such practice is legally obligatory.

Why the other descriptions don’t fit: treating treaty law as unwritten would confuse it with customary law, since treaties are explicitly written. Claiming treaty law governs internal affairs or that customary law only covers maritime boundaries misrepresents how both bodies operate across international relations. Saying treaty law applies only during war and customary law only during peacetime ignores that both can apply in a wide range of situations, including peacetime cross-border cooperation and during conflict.