On 23 January 1878 at the Basilica of Atocha in Madrid, Alfonso married his cousin, Princess Maria de las Mercedes, daughter of Antoine, Duke of Montpensier, but she died within six months of the marriage.

With his marriage to Urraca, queen regnant of Castile and León, in 1109, he began to use, with some justification, the grandiose title Emperor of Spain, formerly employed by his father-in-law, Alfonso VI.

The marriage had been arranged by her father Alfonso VI of León in 1106 to unite the two chief Christian states against the Almoravides, and to supply them with a capable military leader.

Elena Lourie ( 1975 ) suggested instead that it was Alfonso's attempt to neutralize the papacy's interest in a disputed succession — Aragon had been a fief of the Papacy since 1068 — and to fend off Urraca's son from her first marriage, Alfonso VII of Castile, for the Papacy would be bound to press the terms of such a pious testament.

After being widowed in 1327, Alfonso married in February 1329 Eleanor of Castile ( 1308 – 1359 ), who was betrothed to his brother James, who had refused to consummate the marriage.

Alfonso had been betrothed to Maria of Castile ( 1401 – 1458 ; sister of John II of Castile ) in Valladolid in 1408 ; the marriage was celebrated in Valencia on 12 June 1415.

In 1137, Aragon and the County of Barcelona merged in dynastic union by the marriage of Ramon Berenguer IV and Petronilla of Aragon, their titles finally borne by only one person when their son Alfonso II of Aragon ascended to the throne in 1162.

He had accompanied as canon Diego de Acebo, Bishop of Osma on a diplomatic mission to Denmark, to arrange the marriage between the son of King Alfonso VIII of Castile and a niece of King Valdemar II of Denmark.

A year after her marriage to Isabella's brother, Alfonso in 1502, the notorious Lucrezia Borgia became the mistress of Francesco.

Although this was denied, a child was born in that year before Lucrezia's marriage to Alfonso of Aragon.

He condemned King Alfonso IX of León for his marriage to Teresa of Portugal on the grounds of consanguinity.

Following the marriage between Alfonso and Berenguela of Castile, Celestine excommunicated Alfonso and placed an interdict over León.

His father Henry III had demanded the marriage in exchange for ending the war with her brother Alfonso X of Castile.

He arranged that his daughter Maria should wed Alfonso XI of Castile ( 1328 ), but the marriage precipitated the war it was intended to avert, and peace was only restored ( 1330 ) after Queen Isabella had again intervened.

* Alfonso, b. 1184, legitimized through the subsequent marriage of his parents, died before his father.

In 1254, English fears of a Castilian invasion of the English province of Gascony induced Edward's father to arrange a politically expedient marriage between his fourteen-year-old son and Eleanor, the half-sister of King Alfonso X of Castile.

After the election of Theobald I as king of Navarre, his father tried to arrange a marriage for Alfonso with Theobald's daughter, Blanche of Navarre, but the move was unsuccessful.

In the same year Alfonso's sister, Eleanor of Castile, married Henry's heir to the throne, Edward: with this act Alfonso renounced forever all claim to the Duchy of Gascony, to which Castile had been a pretender since the marriage of Alfonso VIII of Castile with Eleanor of England and Gascony.

Violante was ten years old at the time of her marriage to Alfonso ; she produced no children for several years and it was feared that she was barren.

Alfonso almost had their marriage annulled, but they went on to have eleven children:

* Alfonso Fernández de Castilla ( 1242 – 1281 ), also known as el nino, he was señor de Molina and of Mesa through his marriage with Blanca Alfonso de Molina.

In 1175 Pope Alexander III annulled Ferdinand II and Urraca of Portugal's marriage due to consanguinuity.

Author Bernard F. Reilly suggest that rather than a betrothal, the eight-year-old Urraca was fully wedded to Raymond of Burgundy as he almost immediately appears in protocol documents as Alfonso VI's son-in-law, a distinction that would not have been made without the marriage.

Reilly doubts that the marriage was consummated until Urraca was 13, as she was placed under the protective guardianship of a trusted magnate.

However, after Raymond died in 1107, Urraca ’ s father contracted with Alfonso I of Aragon, known as the Battler, for a dynastic marriage with Urraca, opening the opportunity for uniting León-Castile with Aragón.

Urraca protested against the marriage but honoured her late father ’ s wishes ( and the Royal Council's advice ) and continued with the marriage negotiations, though she and her father ’ s closest advisers were growing weary of Alfonso I's demands.

The marriage of Urraca and Alfonso I almost immediately sparked rebellions in Galicia and scheming by her illegitimate half-sister Theresa and brother-in-law Henry, the Countess and Count of Portugal.

Additionally, as Urraca was married to someone many in the kingdom objected to, the queen's son and heir became a rallying point for opponents to the marriage.

By the fall of 1112 a truce was brokered between Urraca and Alfonso with their marriage annulled.

Policies and events pursued by Alfonso VI – namely legitimizing her brother and thereby providing an opportunity for her illegitimate half-sister to claim a portion of the patrimony, as well as the forced marriage with Alfonso I of Aragon – contributed in large part to the challenges Urraca faced upon her succession.

He gave his daughters, Elvira, Urraca and Theresa, in marriage to Raymond of Toulouse, Raymond of Burgundy and Henry of Burgundy respectively.

Urraca also had to contend with attempts by her son ( offspring of her first marriage ), the king of Galicia, to assert his rights.

Because he could not defeat Al-Mansur by arms, Sancho went to Córdoba as an ambassador for his own kingdom, bringing many gifts for the victorious Al-Mansur, making a pact with him and agreeing to give the Muslim his daughter Urraca in marriage.

Perhaps due to his marriage to Urraca, Ferdinand was generous to Afonso, and let him leave.

However, the marriage of Ferdinand II and Urraca was annulled in 1175 by the Pope, the two being second cousins, great-grandchildren of Alfonso VI of León and Castile.

After the dissolution of her marriage, Urraca returned to the court of her father at Valladolid, and died there, aged only 37, nine months after the death of her former husband.

** Alfonso VI of Castile gives her daughter Urraca of Castile in marriage to Raymond of Burgundy together with the fiefdom of Galicia.

** The marriage between Urraca of Castile and Alfonso I of Aragon is annulled.

** Negotiations between Portugal and León result in the marriage of Princess Urraca of Portugal, King Afonso I's daughter, with King Ferdinand II of León.

It has its origins in the marriage of the French crusader Bertrand de Reynel with Elvira Perez de Lara, natural daughter of Queen Urraca of Castile, in the 12th Century.

Fernán was defeated and made prisoner for 3 years until he became reconciled with his sovereign, giving his daughter Urraca in marriage to the king's heir, Ordoño, who afterwards became King Ordoño III.