GUIDO da BAISIO. - Canonist, born into a Bolognese family in Baisio (Reggio Emilia) ca. 1246-56, died in Avignon on 10 Aug. (5 June?) 1313. He studied law under Guido da Suzzaria and Giovanni D'Anguissola. He was chaplain to the bishop of Parma Gera, who later became cardinal and bishop of Sabina; subsequently he is found in Bologna, where in 1296 Boniface VIII appointed him archdeacon of the cathedral chapter.
In Bologna, and perhaps earlier in Reggio, he taught canon law with great renown, first as a free lecturer (until 1301) and then with an official appointment (1301–1304), having as a pupil Giovanni d'Andrea. The period of his teaching in Bologna coincided with his exercise of the archdeaconry, for which he is and has long been cited as the “Archdeacon” by way of annotation. In 1304 he was summoned to Avignon, where Benedict XII appointed him chaplain to the papal court and writer of the so-called contradictory letters.
Among his principal works, the most prominent is a voluminous apparatus on the Decretum, entitled Rosarium Decreti (Strasbourg 1472 and many subsequent editions), composed between 1296 and 1302 with the aim of updating and supplementing, with his own and other contributions—especially extracts from Huguccio—the ordinary gloss on the Decretum itself; indeed, the latter is enriched in later editions with many additions drawn from the Rosarium. G. da B. ranked among the leading commentators on the Liber sextus with his Apparatus seu Glossa seu Commentarius ad Sextum (Milan 1480), which he completed between 1306 and 1311: a much more extensive but less precise work than that of Giovanni d'Andrea, which later became the ordinary gloss. He also wrote a Tractatus de haeresibus et aliis criminibus ad Clementem V, composed on the occasion of the celebrated trial of the Templars accused of heresy; this work survives in manuscript, and Mansi publishes its two principal sections (Mansi, XXVI, 415–26; ibid., Supplement, 357–86).