In scientific work Vetulani was best known for his research on Decretum Gratiani.
The Decretum Gratiani is a collection of Canon law compiled in the twelfth century by a jurist named Gratian.
For a long time he was believed to have been born at the end of the 11th century, at Chiusi in Tuscany.
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Less well-known was the commentary of Simon of Bisignano, which consisted of the Glosses on the Decretum and the Summa Simonis.
Johannes Teutonicus Zemeke (d. 1245) - glossator on the Decretum Gratiani, see Glossa Ordinaria