On September 18, 1964, former King Constantine II of Greece married the Danish Princess Anne-Marie (two weeks after her 18th birthday).
Constantine II was the only son of King Paul and Princess Friederike of Hanover, while Princess Anne-Marie was the youngest daughter of King Frederick IX of Denmark and Princess Ingrid of Sweden.
The couple, third cousins, first met in 1959 when then-Crown Prince Constantine accompanied his parents on a state visit to Denmark. Princess Anne-Marie was just 13 at the time. They met again in 1961, and in 1962, Anne-Marie was a bridesmaid at the wedding of Constantine's older sister, Princess Sophia.
In 1962, Princess Anne-Marie was on holiday with her governess in Norway, where Crown Prince Constantine was attending a yacht racing event, he proposed, she accepted. King Frederick IX initially withheld his consent, as Anne-Marie was only 15 at the time, but eventually relented on the conditions that she finish her education and reach her 18th birthday.