Vicedomino de Vicedominis
Born at Piacenza, he was the cardinal-nephew of Pope Gregory X, his cousin who elevated him on June 3, 1273. From 1257 until 1273 he was archbishop of Aix, then cardinal-bishop of Palestrina. He possibly became dean of the Sacred College of Cardinals in July 1275.
According to the later accounts, attested for the first time in 17th century, in the papal election held after the death of Pope Adrian V he was elected to the papacy on September 5, 1276 and took the name Gregory XI, but having died the following day early in the morning, his election is not recorded as he was never proclaimed. Credibility of this story is contested by modern historians as unsupported by any contemporary source.
See also 
- His name is sometimes referred as Guglielmo Visconti, but this is incorrect.
- Miranda, Salvator. 1998. "Consistory of June 3, 1273."
- This is according to Miranda, Salvator. 1998. "Consistory of June 3, 1273." However, in the bulls of Gregory X that contain subscriptions of the cardinals he is always preceded by Pedro Juliani, which puts into question the accuracy of this statement; cfr. the bulls published in Monumenta Boica, vol. 12, p. 138, and vol. 17 pp. 123-124.
- Cfr. Miranda, Salvator. 1998. "Consistory of June 3, 1273."
- Richard Stapper, Papst Johannes XXI, Kirchengeschichtliche Studien, Munster 1898, p. 34 ff. Dr J. P. Adams Sede Vacante 1276
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