[3] From September 1407 to January 1408, Siena played host to the papal Court of Pope Gregory XII of the Roman Obedience.

[6] When Enea Silvio Bartolomeo Piccolomini, who was born near the town and served as bishop of Siena since 1450, got elected as Pope Pius II in 1458, he soon issued the bull "Triumphans Pastor" (22 April 1459), in which he raised the diocese of Siena to metropolitan status, and assigned to it as suffragans the dioceses of Soano, Chiusi, Massa, and Grosseto.

[7] The Second Vatican Council, in order to ensure that all Catholics received proper spiritual attention, decreed the reorganization of the diocesan structure of Italy and the consolidation of small and struggling dioceses.

Based on the revisions, a set of Normae was issued on 15 November 1984, which was accompanied in the next year, on 3 June 1985, by enabling legislation.

According to the agreement, the practice of having one bishop govern two separate dioceses at the same time, aeque personaliter, was abolished.

This made the combining of Montalcino and Colle di Val d'Elsa under one bishop infeasible.