in the canon law, is a letter of a pope, determining some point or question in the ecclesiastical law. The decretals compose the second part of the canon law. The first genuine decretal, acknowledged by all the learned as such, is a letter of Pope Siricius, written in the year 385, to Himerus, bishop of Tarragona, in Spain, concerning some disorders which had crept into the churches of that country. Gratian published a collection of decretals, containing all the ordinances made by the popes until the year 1150. Gregory IX. in 1227, following the example of Theodosius and Justinian, formed a constitution of his own, collecting into one body all the decisions and all the causes which had served to advance the papal power; and this collection of decretals was called the pentateuch, because it contains five books.