Treaties — Interpretation of — Principles and rules of — Relevance of subsequent conduct of parties — Compatibility of Luxembourg tax with Treaty on Economic Union, July 25, 1921, between Belgium and Luxembourg — Practice of Parties — The law of Luxembourg.

Treaties — Special kinds of — Treaty establishing Economic Union — Prohibition of customs duties between Contracting Parties — Nature of special tax on imports — Compatibility with Treaty on Economic Union, July 25, 1921 — The law of Luxembourg.