These counties came under the rule of the counts of Barcelona, who were Frankish vassals nominated by the emperor of the Franks, to whom they were feudatories (801–988). The earliest known use of the name “Catalonia” for these counties dates to 1117. At the end of the 9th century, the Count of Barcelona Wilfred the Hairy made his title hereditary and founded the dynasty of the House of Barcelona, which ruled Catalonia until 1410.
In 988 Borrell II, Count of Barcelona, did not recognise the new French king Hugh Capet as his king, evidencing the loss of dependency from Frankish rule and confirming his successors (from Ramon Borrell I to Ramon Berenguer IV) as independent of the Capetian crown whom they regarded as usurpers of the Carolingian Frankish realm. At the beginning of eleventh century the Catalan counties suffered an important process of feudalization, partially controlled by the church’s sponsored Peace and Truce Assemblies and by the negotiation skills of the Count of Barcelona Ramon Berenguer I, which began the codification of feudal law in the written Usages of Barcelona, becoming the basis of the Catalan law. In 1137, Ramon Berenguer IV, Count of Barcelona decided to accept King Ramiro II of Aragon‘s proposal to marry Queen Petronila, establishing the dynastic union of the County of Barcelona with the Kingdom of Aragon, creating the Crown of Aragon and making the Catalan counties that were united under the county of Barcelona into a principality of the Aragonese Crown.
In 1258, by means of the Treaty of Corbeil, James I of Aragon King of Aragon and Count of Barcelona, king of Mallorca and of Valencia, renounced his family rights and dominions in Occitania and recognised the king of France as heir of the Carolingian Dynasty. The king of France, Louis IX, formally relinquished his claims of feudal lordship over all the Catalan counties, except the County of Foix, despite the opposition of the king of Aragon and count of Barcelona. This treaty confirmed, from French point of view, the independence of the Catalan counties established and exercised during the previous three centuries, but also meant the irremediable separation between the geographical areas of Catalonia and Languedoc.
As a coastal territory, Catalonia became the base of the Aragonese Crown’s maritime forces, which spread the power of the Aragonese Crown in the Mediterranean, and made Barcelona into a powerful and wealthy city. In the period of 1164–1410, new territories, the Kingdom of Valencia, the Kingdom of Majorca, Sardinia, the Kingdom of Sicily, Corsica, and, briefly, the Duchies of Athens and Neopatras, were incorporated into the dynastic domains of the House of Aragon. The expansion was accompanied by a great development of the Catalan trade, creating an extensive trade network across the Mediterranean which competed with those of the maritime republics of Genoa and Venice.
The domains of the Aragonese Crown were severely affected by the Black Death pandemic and by later outbreaks of the plague. Between 1347 and 1497 Catalonia lost 37 percent of its population. In 1410, King Martin I died without surviving descendants. Under the Compromise of Caspe, Ferdinand from the Castilian House of Trastámara received the Crown of Aragon as Ferdinand I of Aragon. During the reign of his son, John II, social and political tensions caused the Catalan Civil War (1462–1472).
Ferdinand II of Aragon, the grandson of Ferdinand I, and Queen Isabella I of Castile were married in 1469, later taking the title the Catholic Monarchs; subsequently, this event was seen by historiographers as the dawn of a unified Spain. At this time, though united by marriage, the Crowns of Castile and Aragon maintained distinct territories, each keeping its own traditional institutions, parliaments, laws and currency. Castile commissioned expeditions to the Americas and benefited from the riches acquired in the Spanish colonization of the Americas, but, in time, also carried the main burden of military expenses of the united Spanish kingdoms. After Isabella’s death, Ferdinand II personally ruled both kingdoms.