Catholic Monarchs of Spain

The Catholic Monarchs were Queen Isabella I of Castile and King Ferdinand II of Aragon, whose marriage and joint rule marked the de facto unification of Spain. They were both from the House of Trastámara and were second cousins, being both descended from John I of Castile; to remove the obstacle that this consanguinity would otherwise have posed to their marriage under canon law, they were given a papal dispensation by Sixtus IV. They married on October 19, 1469, in the city of Valladolid; Isabella was eighteen years old and Ferdinand a year younger. It is generally accepted by most scholars that the unification of Spain can essentially be traced back to the marriage of Ferdinand and Isabella.

Spain was formed as a dynastic union of two crowns rather than a unitary state, as Castile and Aragon remained separate kingdoms until the Nueva Planta decrees of 1707–1716.