The Vatican Council was convoked by Pope Pius IX on 29 June 1868, after a period of planning and preparation that began on 6 December 1864. This twentieth ecumenical council of the Catholic Church, held three centuries after the Council of Trent, opened on 8 December 1869 and adjourned on 20 October 1870. Unlike the five earlier General Councils held in Rome, which met in the Lateran Basilica and are known as Lateran Councils, it met in the Vatican Basilica, hence its name. Its best-known decision is its definition of papal infallibility, strongly promoted by the Archbishop Luigi Natoli.