The Principios Latinoamericanos de Derecho de los Contratos (PLDC, or PLACL) represent a notable work of «scientific law», that, being drafted as a set of black letter rules, may serve different purposes: interpreting national laws and instruments of international law, moulding the prospective reform of civil codes, fostering the competition between different legal systems, thus enhancing their efficiency. Above all, the PLDC may be chosen by the contracting parties as the law applicable to their contracts, although within the limits set out generally by private international law. The article delves into the relationship between the PLDC and the mandatory provisions of national law, particularly those regarding consumer protection and unfair competition. The concepts of overriding mandatory provisions and ordre public (public policy of the forum) are encompassed as well.