Titular sees are dioceses that no longer functionally exist, often because the territory was conquered by Muslims or because it is schismatic.
[1] During the Muslim conquests of the Middle East and North Africa, some bishops fled to Christian-ruled areas.
Formerly, when bishops fled from invading Muslims, they were welcomed by other churches, while preserving their titles and their rights to their own dioceses.
[2] These bishops did not reside in the sees whose titles they bore, nor could they exercise any power over them,[3] and are not entrusted with their care.
Besides those who were endowed with the episcopal character, in order to assist the local bishops there were those who had been driven from their dioceses by infidels or by heretics, or who for other reasons could not reside in the places to which they had been appointed.
The spread of Islam through Muslim conquests in Asia and Africa was responsible for hundreds of abandoned sees.
In the 16th century the Holy See inaugurated the policy of consecrating nuncios and other prelates, delegated to represent the Pope in his relations with the different nations, so that they would be equals with the diocesan bishops of the countries in which they were ambassadors.
Leo XIII, through a Congregation for the Propagation of the Faith decree, in 1882, abolished the phrase in partibus infidelium and ordered that future appointments should be made as "titular bishops".
[5] The custom, when Boudinhon wrote his article, was to join to the name of the see that of the district to which it formerly belonged, or else merely to say "titular bishop".
[8] Although it did not claim to be perfect, it contained the names of the sees and the bishops who had held the titles as far back, in some cases, as the 14th century.
This change too is reflected in editions of the Annuario Pontificio of the period, which include information on renunciation by retired and coadjutor bishops of titular sees to which they had been appointed.
[9] The crusading William IV, Count of Nevers, dying in the Holy Land in 1168, left the building known as the Hospital of Panthenor in the town of Clamecy in Burgundy, together with some land, to the Bishops of Bethlehem, in case Bethlehem should fall under Muslim control.
[12] The Roman Catholic Church lists as titular sees, former diocese that no longer function, several in the Aegean Islands.