Gratian

The Decretum Gratiani, also known as the Concordia discordantium canonum or Concordantia discordantium canonum or simply as the Decretum, is a collection of canon law compiled and written in the 12th century as a legal textbook by the jurist known as Gratian. It forms the first part of the collection of six legal texts, which together became known as the Corpus Juris Canonici. It was used as the main source of law by canonists of the Roman Catholic Church until the Decretals, promulgated by Pope Gregory IX in 1234, obtained legal force, after which it was the cornerstone of the Corpus Juris Canonici, in force until 1917.