A titular bishop is a bishop holding the title of a see which no longer politically exists or no longer has any Orthodox Christian flock. Titular bishops usually serve as assistants to ruling bishops, similar to an auxiliary bishop but not holding a title which is within the territory of another bishop (which technically makes auxiliary bishops uncanonical). The canonical origin of the titular episcopacy was in the recognition of the Ecumenical Councils of the legitimacy of bishops who could not reach their sees, due usually to the invasion of Muslims.