GREGORY X. (_Tebaldo Visconti_), pope from the 1st of September 1271, to the 10th of January 1276, was born at Piacenza in 1208, studied for the church, and became archdeacon of Liege. The eighteen cardinals who met to elect a successor to Clement IV. were divided into French and Italian factions, which wrangled over the election for nearly three years in the midst of great popular excitement, until finally, stirred by the eloquence of St Bonaventura, the Franciscan monk, they entrusted the choice to six electors, who hit on Visconti, at that time accompanying Edward of England on the crusade. He returned to Rome and was ordained priest on the 19th of March 1272, and consecrated on the 27th. He at once summoned the fourteenth general council of the Catholic Church, which met at Lyons in 1274, with an attendance of some 1600 prelates, for the purpose of considering the eastern schism, the condition of the Holy Land, and the abuses in the church. The Greeks were persuaded, thanks to St Bonaventura, to consent to a union with Rome for the time being, and Rudolph of Habsburg renounced at the council all imperial rights in the States of the Church. The most celebrated among the many reform decrees issued by Gregory was the constitution determining for the first time the form of conclave at papal elections, which in large measure has remained ever since the law of the church. Gregory was on his way to Rome to crown Rudolph and send him out on a great crusade in company with the kings of England, France, Aragon and Sicily, when he died at Arezzo on the 10th of January 1276. He was a nobleman, fond of peace and actuated by the consciousness of a great mission. He has been honoured as a saint by the inhabitants of Arezzo and Piacenza. His successor in the papacy was Innocent V. The registers of Gregory X. have been published by J. Guiraud in the _Bibliotheque des ecoles francaises d'Athenes et de Rome_ (Paris, 1892-1898). See K. J. von Hefele, _Conciliengeschichte_, vol. 5, 2nd edition (1873-1890); H. Finke, _Konzilienstudien z. Gesch. des 13ten Jahrhunderts_ (Munster, 1891); P. Piacenza, _Compendia della storia del b. Gregorio X, papa_ (Piacenza, 1876); F. Gregorovius, _Rome in the Middle Ages_, vol. 5, trans. by Mrs G. W. Hamilton (London, 1900-1902); H. Otto, _Die Beziehungen Rudolfs von Habsburgs zu Papst Gregor X._ (Innsbruck, 1895); A. Zisterer, _Gregor X. u. Rudolf von Habsburg in ihren gegenseitigen Beziehungen_ (Freiburg i. B., 1891); F. Walter, _Die Politik der Kurie unter Gregor X._ (Berlin, 1894); A. Potthast, _Regesta pontif. Roman._ vol. 2 (Berlin, 1875); W. Norden, _Das Papsttum und Byzanz_ (Berlin, 1903); J. Loserth, "Akten uber die Wahl Gregors X." in _Neues Archiv_, xxi. (1895); A. von Hirsch-Gereuth, "Die Kreuzzugspolitik Gregors X." in _Studien z. Gesch. d. Kreuzzugsidee nach den Kreuzzugen_ (Munich, 1896). There is an excellent article by Carl Mirbt in Hauck's _Realencyklopadie_, 3rd edition.