INTRODUCTION

• 1. Article 9 of the American Convention enshrines a basic principle of criminal law, the Latin wording of which is nullum crimen et nulla poena sine lege. This constitutes “the principle of legality” explained by the Court in De la Cruz Flores as follows: “The definition of an act as an unlawful act and the determination of its legal effects must precede the conduct of the individual who is alleged to have violated it; because, before a behavior is defined as a crime, it is not unlawful for penal effects.” This is a fundamental principle for the protection of human rights, leading the Convention to establish it as an international State obligation that cannot be suspended, even in an emergency situation.

• 2. This provision also enshrines the principle of the most favorable law, which means that a convict must receive a lesser punishment when a lesser punishment is established by law after the commission of a crime. Like Article 8, Article 9 of the Convention applies to all criminal convictions, regardless of significance or scope. It does not apply to procedural law, because that law applies at the moment a procedural act is carried out, independently of the date of the commission of the offense. Regarding procedural law, in the case of Liakat Ali Alibux the Court relied heavily on case law of Latin American States and of the European Court. The Inter-American Court observed that the principle of legality, in the sense that a law existed prior to the commission of a crime, does not apply to regulations governing procedure. However, it accepted that an exception to this rule is possible if the procedure had “an impact on the classification of acts or omissions that at the time of commission were not criminal” or if it leads to “the imposition of a penalty that is more serious than the one in place at the time of the commission of the crime.” In the case subject to the decision, there had been no violation of Article 9.

NULLUM CRIMEN SINE LEGE

• 3. The law that Article 9 refers to must be a law both formally and materially.