Decretals are collections of letters containing papal rulings of local or universal application, often made in response to an appeal and frequently relating to matters of canonical discipline. Decretals may be illuminated with scenes germane to the text or with scenes designed to relieve the text, such as bas-de-page scenes narrating saints' lives or amusing scenes from daily life and grotesques. Copies of decretals were often required by ecclesiastical and civil authorities or for study purposes in universities, notably those specializing in law, such as the university at Bologna. The earliest decretals were simple collections of papal letters. They were often included in collections of canon law and as early as the fifth century were themselves arranged in collections. Among the most notable are: the Collectio Dionysiana of Dionysius Exiguus, compiled c. 514; Pope Hadrian I's collection, sent to Charlemagne in 774 (Collectio Dionysio-Hadriana), which became the authoritative Frankish text on canon law; the Spanish sixth-century collection associated with Isidore of Seville (Hispana collectio); the Fake Decretals of Pseudo-Isidore, compiled in France around 850; the ninth-century Italian Anselmo dedicata collectio; the tenth-century Collectio of Abbo of Fleury; Regino of Prüm's collection of canon laws made in 906; the Decretum of Burchard of Worms, c. 1012, an influential collection designed to promote church reorganization; and the late eleventh-century Decretum of Ivo of Chartres. Gratian's Decretum of c. 1140 summarized older letters and conciliar decrees and became the most important law book of the twelfth century. It marked the end of the traditional form of collection, giving rise to the greatest period of legal scholarship in the Roman Church. Later collections include the Quinque compilationes antiquae; the Decretals of Pope Gregory IX of 1234, an important new edition of canon law; the Liber sextus of Pope Boniface VIII of 1298; and the Constitutiones Clementinae of Pope Clement V of 1317. Decretals generated a number of commentaries, which often appeared as glosses.
Michelle Brown. Understanding Illuminated Manuscripts (Malibu, CA: J. Paul Getty Museum in association with the British Library, c1994).