During the Roman Republic and the Empire, today's Spain was a province under the jurisdiction of the Empire at that time, and these lands were incorporated into the territory of Rome when Caesar made an expedition to Gaul. After that, Spain fell into a loose feudal state, and many kingdoms, principalities, lords and warlords survived until the Moors were reunited. Charlemagne tried to merge Spain into the Frankish Empire but failed. His nephew Roland was killed because of another expedition. As a unified nation-state, Spain was unified with the three great powers of Iberian Peninsula-Leon, Castilla and Aragon only after Ferdinand II and Isabella I got married.

Officially, the Spanish word "Espa" didn't appear in the title of Bourbon monarch until 1837. Isabella II, who was in power at that time, decided to replace her original lengthy title "Queen of Castilla, Leon, Aragon" with "Spain". But it is worth noting that "Spain" at this time is a plural word in Latin-"Reina de las Espa". It was not until 1874 that Spain appeared as a singular noun in the title of Spanish royal family. However, Spain's "Hispania" as the name of the kingdom appeared as early as16th century ago. The Latin "rex Hispaniarum (King of Spain)" was used to replace the formal and lengthy title on coins minted during the reign of Philip II. The exact date of the emergence of "Spain" is uncertain, but the following four time points can be regarded as the beginning of the unified kingdom of Spain:

1479, King Ferdinand of Castile succeeded his father Aragon to the throne, and Spain has been ruled by a single family ever since.

15 16 years, the future holy Roman emperor Charles V inherited his grandfather's Aragon throne, won the title of king in Castilla, and ruled with his mother. According to the cloud, coins engraved with "Carolus and Johanna, Lei Gesi Hispani Auraru and Indian Rum" are made in Mexico.

1555, Queen Juana died, and Charles V became the only king of Castile. At this time, Spain has not been formally unified, and Portugal is an independent country.

1580, a crisis of succession to the throne occurred in Portugal. Castilla, son of Charles V, and Philip II, king of Aragon, won the Portuguese crown. At this time, some people think that the kingdom of Castilla and Aragon and the kingdom of Portugal form the main confederation as a single kingdom of Spain. 1640, Portugal broke away from the main confederation and established the Braganca family as king. Other kingdoms continued to be ruled by the Habsburg dynasty in the name of Spain. But Spain is still not unified, and it still exists in the form of a confederation of several kingdoms.

Therefore, the signatures of successive Spanish monarchs do not need any titles of their own, and they are often just a sentence, "I, the king (Yo El Rey)" or "I, the queen (Yo La Reina)".