The Feeral Rules of Evidence (FRE) govern the admission of facts by which parties in the federal courts of the United States may prove their cases. They were the product of protracted academic, legislative, and judicial examination before they were formally promulgated in 1975. For that reason, they serve as a model for the rules of evidence used in state court systems, as well as a teaching tool in American law schools. Although they were formally adopted by the U.S. Congress, states are free to adopt different rules of evidence. Because they govern the initial presentation of evidence in a trial, the Rules are primarily aimed at federal trial courts rather than appellate courts. Appellate courts do, however, monitor the application of the rules to ensure consistent application and coherent development of the federal common law of evidence.