William I of Württemberg
On 24 January 1816, in Saint Petersburg, he married his first cousin, Grand Duchess Catherine Pavlovna of Russia (1788–1819), daughter of Emperor Paul I of Russia (1754–1801) and Princess Sophie Dorothea of Württemberg (1759–1828). The couple had two children:
Frederica of Hanover
Prince Paul of Greece, her mother's paternal first cousin, proposed to her during the summer of 1936, while he was in Berlin attending the 1936 Summer Olympics. Paul was a son of King Constantine I and Frederica's grand-aunt Sophia. Their engagement was announced officially on 28 September 1937, and Britain's King George VI gave his consent pursuant to the Royal Marriages Act 1772 on 26 December 1937. They married in Athens on 9 January 1938. Frederica became Hereditary Princess of Greece, her husband being heir presumptive to his childless elder brother, King George II.