It is a rather messy history that has to do with the Three-Chapter Controversy, the Patriarch of Aquilea and the Patriarch of Grado. More or less Aquilea proclaimed themselves a Patriarchate and went into schism in the 6th century. The Patriarchate was moved to Grado in the early 7th century and soon entered back into communion with Rome. But, in Aquilea, a rival Patriarchate was established. By the end of the 7th century, the Patriarchate of Aquilea entered into communion with Rome too. Both Grado and Aquilea were allowed to maintain the title of "Patriarch" though. They were honorary titles. Due to politics, the rise of the Venetian Republic, etc., Grado was superseded by Venice, and Aquilea would later be dissolved into the Archdiocese of Udine and Archdiocese of Gorizia. Aquilea is now a titular see.