Article 45 Montreal Convention
Article 45 of the Montreal Convention (Convention For the Unification of Certain Rules for International Carriage by Air) is titled Addressee of Claims. It is under Chapter V (Carriage by Air Performed by a Person other than the Contracting Carrier).
In relation to the carriage performed by the actual carrier, an action for damages may be brought, at the option of the plaintiff, against that carrier or the contracting carrier, or against both together or separately. If the action is brought against only one of those carriers, that carrier shall have the right to require the other carrier to be joined in the proceedings, the procedure and effects being governed by the law of the court seized of the case.
See also: Article 44 Montreal Convention