The archdiocese of Zaragoza has a peculiarity that is unique in the world, which is also shown physically in the surroundings of the basilica del Pilar: it has a cathedral chapter and two cathedrals. Contrary to what happens in other cities like Cadiz or Salamanca, it is not a question of an old cathedral and another new cathedral that comes to substitute, in functions to the old one... but of two cathedrals of its own.
The Cathedral of the Savior, known as La Seo, and the Basilica del Pilar, which is also a cathedral. The history dates back to the time of the Muslim domination of the city in which two churches: Santa María la Mayor (which later became the Basilica del Pilar) and the Santas Masas (Santa Engracia) hosted the Christian worship of the city. With the reconquest, the old main mosque of the city was consecrated as a cathedral and dedicated to the Savior in 1118. In 1121, the chapter of canons of the Savior was created. Shortly thereafter, in 1299, the church of Santa Maria became a collegiate church of canons regular and disputes began between the two bodies of canons. While the members of the collegiate church of Santa María del Pilar defended their position as the first Marian temple, the canons of El Salvador defended their privilege as episcopal see.
The conflict continued over time and reached such a point that, in the 17th century, Pope Clement X promulgated the Bull of Union (1676), which "united the two churches of El Salvador and El Pilar, making them one Metropolitan Church, and one Chapter". This Bull is still in force and, at present, in an unprecedented manner in the rest of the world, the Metropolitan Chapter of Zaragoza is constituted by a single chapter with two residences (La Seo and El Pilar), which are exchanged on April 1 of each year.
It was in 1948 when Pius XII granted the title of Minor Pontifical Basilica to the Cathedral where the Blessed Virgin of Pilar is venerated, thus turning the Pilar into a basilica-cathedral, as it is known today.
