Decretals (Epistolae decretales) is the name that is given in Canon law to those letters of the pope which formulate decisions in ecclesiastical law. They are generally given in answer to consultations, but are sometimes due to the initiative of the popes. These furnish, with the canons of the councils, the chief source of the legislation of the church, and formed the greater part of the Corpus Iuris Canonici before they were formally replaced by the Codex Iuris Canonici of 1917. However, Cardinal Pietro Gasparri led the papal commission for the revision of canon law and later on published a guide to the fonts used in the 1917 code, many canons in this code can easily be retraced in their relationship to and dependency on medieval decretals as well as Roman law.