The Kings and Queens of Spain

Spain was first unified by the marriage of FERDINAND AND ISABELLA, who pursued the military unification of Spain after their marriage in 1474. However, it was not until 1556 that Spain truly came to be ruled as a single monarchy and between 1474 and 1556, it technically remained a dual monarchy, with power being shared between whichever members of the royal house inherited power in Aragon and Castile, the two kingdoms first united by Ferdinand and Isabella.

FERDINAND II, sometimes known as Ferdinand the Catholic, was Ferdinand V in Castile. He was King of Aragon, Sicily, the Naples and Sardinia. He was styled King Ferdinand V of Castile jure uxoris between 1474 and 1503. He ruled as his wife's partner from 1474 to 1479 and then they co-ruled Spain together between 1479 and 1503. He was married to ISABELLA I, sometimes known as Isabella the Catholic. She ruled from 1474 to 1503. After her death, Ferdinand technically shared power in a royal triumvirate with his daughter, Juana, and her husband, Philip.

JUANA, sometimes known as Juana la loca or Joanna the Mad, was married to Philip the Handsome, Archduke of Austria, who shared power with her in Castile as King PHILIP I. Philip I co-ruled with Juana and de facto with his father-in-law, Ferdinand II, from 1504 to 1506. After his death, Juana's technical co-ruler was her son, Charles. (See below.) However, in reality all power was shared between her son and father, since Juana was subsequently incarcerated on grounds of her allegedly unstable mental health. She continued to rule as Queen of Castile, in name only, until her death in 1555.