Liber Extra
Canon lawyers of the twelfth and thirteenth centuries taught canon law by commenting on the "Decretum of Gratian" and on the various collections of the Decretals. The "margaritae" were developed as collections to aid memory. They arranged the more important propositions, denominated "résumés", and axioms in alphabetical order or by subject matter, including mnemonic verse. Many of these "margaritae" have been preserved, but not all of their authors are certainly known. Some of them have been printed with the "Decretum" or the "Decretals of Gregory IX".