A royal mistress is the historical position of a mistress to a monarch or an heir apparent.
 Some mistresses have had considerable power, such mistresses have sometimes been referred to as the "power behind the throne".
 The prevalence of the institution can be attributed to the fact that royal marriages were until recent times conducted solely on the basis of political and dynastic considerations, leaving little space for the monarch's personal preferences in the choice of a mate.
Agnès Sorel (1422 – 9 February 1450), known by the sobriquet "Dame de beauté", was a favourite mistress of King Charles VII of France, by whom she bore three daughters.
 She is considered the first officially recognized royal mistress.
 She was the subject of several contemporary paintings and works of art, including Jean Fouquet's "Virgin and Child Surrounded by Angels".
Hedvig Ulrika Taube (31 October 1714 – 11 February 1744), also Countess von Hessenstein was a Swedish courtier and countess, a Holy Roman countess of the Empire, and royal mistress to king Frederick I of Sweden from 1731 to 1744.
 She is regarded as the only official royal mistress in Swedish history.
Stella de’ Tolomei (d. 1419), was an Italian courtier and royal mistress.
 She was the official royal mistress of Niccolò III d'Este, Marquis of Ferrara for many years and the mother of several of his acknowledged illegitimate children.
Claude de Vin des Œillets, known as Mademoiselle des Œillets (] ; Provence 1637 – Paris, 18 May 1687), was a mistress of King Louis XIV of France and the companion of the official royal mistress and favourite Madame de Montespan.
 She was known for her involvement in the famous Affair of the Poisons (1679–1680).
Eliza Meek (March 3, 1832 – February 8, 1888) was the daughter of Captain John Meek, an early American settler of the Kingdom of Hawaii.
 In her early youth, she was renown for her equestrian skills on her father's land.
 She later became the royal mistress of King Lunalilo and formed a contentious relationship with Queen Emma of Hawaii and was rumored to be the main obstacle between a possible marriage between the two.
 Along with a group of other members of the royal court, Eliza accompanied the king to Kailua-Kona during his last illness and remained by Lunalilo's side until his death from tuberculosis on his return to Honolulu on February 3, 1874.
 Little is known about her later life; Eliza was financially well-off until her death on February 8, 1888.
Frederikke Benedichte Dannemand, born as "Bente Frederikke Mortensdatter Andersen Rafsted" (6 August 1790 – 23 December 1862) is known in history as the royal mistress of King Frederick VI of Denmark.
Biette Cassinel (1340-1380), was the royal mistress of Charles V of France, from 1360 until 1380.
 She has been referred to as the first official mistress of a French monarch.
Amalie Sophie Marianne von Wallmoden, Countess of Yarmouth, born Amalie von Wendt (1 April 1704–19 or 20 October 1765) was the principal mistress of King George II from the mid-1730s until his death in 1760.
 Born into a prominent family in the Electorate of Hanover, and married into another, in 1740 she became a naturalised subject of Great Britain and was granted a peerage for life, with the title of "Countess of Yarmouth", becoming the last royal mistress to be so honoured.
 She remained in England until the death in 1760 of King George II, who is believed to have fathered her second son, Johann Ludwig, Reichsgraf von Wallmoden-Gimborn.
 She returned to Hanover for the rest of her life, surviving the king for nearly five years.
Frederick VI (Danish and Norwegian: "Frederik"; 28 January 17683 December 1839) was King of Denmark from 13 March 1808 to 3 December 1839 and King of Norway from 13 March 1808 to 7 February 1814.
 From 1784 until his accession, he served as regent during his father's mental illness and was referred to as the "Crown Prince Regent" ("kronprinsregent").
 For his motto he chose "God and the just cause" (Danish: "Gud og den retfærdige sag" ) and since the time of his reign, succeeding Danish monarchs have also chosen mottos in the Danish language rather than the formerly customary Latin.

Question: Frederikke Dannemand was the royal mistress of which King?
Answer: Frederick VI of Denmark