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It is a symbolic title only, the sole constitutional function of the holder being similar to a prince consort, which is the male equivalent of a queen consort. There is a gender asymmetry in the titles because, under male primogeniture, the royal throne usually goes to a male heir, and passes to a female only when there is no male heir. The title of king usually refers to a reigning king, a sovereign monarch; while the title of queen usually refers to a queen consort, the non-reigning wife of a reigning king.
Mary, Queen of Scots (reigned 1542–1567) married Henry Stuart, Lord Darnley, the eldest son of the Earl and Countess of Lennox in July 1565. Darnley was a great-grandson of King Henry VII of England and Mary's first cousin, and he was considered to have a strong claim to the Scottish throne. On the evening before their marriage, Mary proclaimed Darnley "King of Scots", a title that she could not legally grant him without the consent of Parliament, but which was never formally challenged. However, this title did not grant him any automatic right of rule or of succession to the throne if Mary should die. For that to happen, it was necessary that Mary grant him the Crown Matrimonial of Scotland, which never happened. Mary's marriage to Darnley rapidly became unhappy and, despite Darnley's constant demands for the Crown Matrimonial, Mary never gave it to him. If she had, Darnley would have inherited the throne of Scotland if Mary (and their children, if any) had predeceased him. Under that scenario, if Darnley had then remarried and had children with his new queen, he would have started a new dynastic line and those children also would have been the legitimate heirs to the Scottish throne.
Queen Victoria (reigned 1837–1901) wanted to make her husband Albert king consort, but the British government refused to introduce a bill allowing it, as Albert was a foreigner. She instead gave him the title of Prince Consort in 1857.
In the United Kingdom, there is no automatic right of the consort of a queen to receive any title, as with any husband of a suo jure peeress. Queen Elizabeth II (acceded 1952) did not create her husband Prince Philip, Duke of Edinburgh a Prince of the United Kingdom until 1957, five years after her accession. He has never been formally designated Prince Consort or King Consort.
In the event that a king consort were to outlive his wife, he would remain a king in the same way that a queen dowager remains a queen after being a consort. He would retain the style His Majesty and would remain a member of the royal house. This was the case with Francis, Duke of Cádiz, who remained King even after his wife's reign was over. Also if a King consort produced an heir to the throne and his son or daughter is the new king or queen regnant, the king could also be given the title king father - like that of a queen mother, which is normally given to queens consort after they become dowager queens. This is similar to when King Norodom Sihanouk of Cambodia abdicated and was later given the title HM King-Father Norodom Sihanouk.